Bookstores are many, many things. One of them is a morass of details. Sometimes we will hear from a vendor about a missing payment, and it’s clear the vendor is under the impression that we have actively chosen not to pay their bill. While it’s true that invoice juggling can happen with retailers (there are certainly lean cash-flow months and full cash-flow months), for us, this happens only with invoices for hundreds (or thousands) of dollars, when we are waiting to be paid by a school or library before we can cut a large check.
Compounding this issue is that small indie booksellers may not have dedicated bookkeepers; bill-paying is just one of the hats bookstore owners wear. An important hat, of course, but one among many. On top of that is the fact that some vendors (the smaller ones) often just send a single invoice instead of the additional monthly statements sent by larger vendors. In an ideal world, one invoice would be enough. We do understand that it is costly and time-intensive to send more than one bill through the mail. Except. We get a lot of bills. Here is a couple of months’ worth of invoices, statements, and credits from vendors (Josie took some home, so this is not even all of them!):

As tall as the paper invoice stack is, the email “piles” are even bigger. Floods of emails pour into the bookstore every day. We deal with hundreds of vendors; I don’t even want to calculate how many invoices and statements that adds up to over the year.
For booksellers, the vast majority of bill-paying and email reading has to happen outside of store hours. During the work day, we are on the store floor helping customers, we are receiving orders and stocking the shelves and calling customers to let them know their books have arrived, we are answering phones and tracking down obscure requests and missing shipments, we are meeting with sales reps to order seasons of new books and sidelines, we are processing returns and damages, we are reorganizing sections and reshelving books customers have looked at, changing displays, and planning events.
So in case you’re a publisher or a sidelines vendor or an individual author waiting to be paid by a bookstore, this might give you a window into why your check hasn’t arrived just yet. We’re good for it, just running to catch up.