Hopefully the OP  will return and let us know the circumstances.

Different sellers?    Different mattresses ?  Different delivery days ?  Different orders?

On the off chance it helps, I'll add this

How to return multiple items

 Multiples of the same item  (already mentioned) – You can return some or all of them, but you can only open one return for the order. For example, if you bought five t-shirts from one listing, you can return two, but you won't be able to return the other three later.

Combined purchases from the same seller – You can return each item individually. For example, you bought three different books from the seller's store and paid for them together at checkout. You need to open a separate request for each book, because they came from different listings.