I just wanted to chime in here and say I did a little research on the Amazon Pending problem because we also got bit by it when we sold out of an item while the pending sale sat at Amazon for 4 days.
A pending sale according to Amazon is one where they are verifying payment. It usually takes place within 2 hours (much sooner when things aren’t so busy). However, if the buyer’s address has changed, or the credit card expiration date has run out, Amazon starts emailing their customer about the issue and the order stays in pending. Amazon allows up to 7 days for the problem to be remedied before they cancel the order, but most become actual sales.
Amazon states that you MUST manage your inventory even though you have a long term pending order. It is still considered a sale to their customer and you must reserve the inventory for that sale until the pending order is remedied.
Since there are no emails about pending orders to the merchant and Nitrosell can’t have access to them, the only thing I can suggest is watching your pending order bucket in your Amazon portal every day and see what is in there.
Just thought I would throw in my 2 cents on this one.