This is a general rundown of how Amazon split shipments work. This will focus on individual product shipments rather than case packed.
The first factor that affects shipments is using Amazon’s FBA Inventory Placement Service. This way your shipments rarely if ever get split into multiple shipments. It seems the shipment will usually go to the distribution center where the main split would have gone otherwise. Without inventory placement the batch or shipping plan will almost always be split into at least 3 shipments to different distribution centers. It’s also possible the batch isn’t split at all. That’s kind of rare and difficult to determine why it happens without inventory placement. How the batch is split is mainly determined by the type of products and the quantity of each in the batch. Batches/plans will still be split between the following groups: Apparel, Jewelry, Shoes, Media, Inventory tracked with a manufacturer barcode (commingled), Oversize items, Amazon prep required, Amazon labeling required, Hazardous materials.
A large factor that affects shipments is whether they are commingled or seller labeled. Commingling may be beneficial for case packed units, but not individual units. We highly recommend you avoid commingling individual units. Commingling uses the manufacturer’s UPC barcode to track the product at Amazon’s warehouses. Seller labeled products use a unique code for that sku/product and your individual seller account. One main reason to avoid commingling is if you make a sale and the customer receives a bad product sent by another seller. You’ll be the one penalized instead of the offending seller. If the UPC is not tied to the Amazon listing you want then the product will be declared incorrect or missing when it arrives at Amazon and that’s if the associate checks it in rather than losing it.
Another reason to not commingle is if you or we use a service like Inventory Lab to list your products. Due to an incompatibility with the Amazon API, seller labeled units cannot be combined in the same shipment as commingled units in shipments created by Inventory Lab. They can be combined if the Inventory Lab batch is exported as a shipping plan and processed through Seller Central. Thus bypassing all the useful features of creating box contents in Inventory lab. This also increases the possibility of making a mistake when boxing the products. The alternative is to have the commingled shipped separately from seller labeled units even though they are going to the same distribution center. That adds to the shipping costs and eats into profit. There is a positive affect of commingling though. That is you can make a sale before your product has arrived because you’ll be drawing from the pool already at the distribution center.
Hazmat units are always sent by themselves and can be split even further if the batch is kind of small relative to how many hazmat units are in it. Hazmat shipments can’t use Amazon’s partnered carrier services. So they are usually processed through the carrier directly without the deep discount of the partnered carrier service. The best way to avoid additional splits is to include them in a large batch with the hazmat less than 5% of the total units. It’s also important to send hazmat units in as few batches as possible to minimize the higher shipping costs.
Oversized units (greater than 15 inches) are also usually separated and sent to random specialized distribution centers, possibly across the country. These do use discounted partnered carriers unlike hazmat. Including the oversized units in a large batch will minimize multiple units getting split even further.
Shoes and shoe related products are mostly sent to Maryland and could be split to Indiana or possibly other locations. The shipping is relatively costly with high “dimensional weight”. They take some time to travel from Oregon to across the country.
Standard items are almost always split into 3 shipments. Four or more is possible but rare. It’s always different ratios from one batch to the next. Usually two shipments are small. The smallest can be between half to exactly the same as the other small shipment. Together they tend to be less than 25% of the total batch. As an example, if the whole batch is ten equal volume boxes, then the smallest shipment could be in 1-2 boxes, the next shipment in 1-3 boxes, and the large shipment in 5-8 boxes. In my experience the largest shipment will usually, but not always, go to the same distribution center batch after batch. Here in Oregon that center is SMF3 in Stockton, CA. The other two shipments go randomly throughout the country. One usually goes somewhere between the west coast to the midwest. The third usually goes between the midwest and east coast. Many times the two shipments go to the east coast. It can be possible to roughly know where they might go depending on the kinds of products or styles. People use different products in different regions and have different tastes too. Amazon’s algorithm can take that into account. A new distribution center can be chosen multiple times for a while too.
Deciding when to finalize a batch to create a shipment can be a game of chance and skill. There’s a constant balance between spatial volume, unit counts, including hazmat and oversized, and doing quick turnarounds. Quick turnarounds and multiple hazmat and oversized units make for higher shipping costs. Delaying too long can lead to missed opportunities or lower buy box prices from competing sellers.
I’m not an Amazon seller or expert on lead times. My humble advice is to have a lead time of at least 4 weeks before an event or season. That way all the product is in stock and ready to go when customers start looking online for those items. Three weeks could be cutting it close. Less than that is usually too late. Especially when multiple sellers are flooding their prep centers and the distribution centers with product at the same time. Of course it’s a risk and balance between tying up capital versus having stock arrive after the sales are drying up.