Invoice product order... help!

I'm using Zen Cart to sell very custom merchandise and need it to list products on the invoices and emails in the order they were added to the cart. For some reason I'm usually able to do this just fine when I test it but several of my site visitors have come on to the store and there is no rhyme or reason to how their items are listed on the invoices and emails so I'm having a hard time determining what they've ordered.... Basically I sell jewelry with accessory products and these orders need to help me determine which accessories go with each jewelry item based on the order they were put into the cart. Where would I find more info about this? Like I said, it seems to do it this way most of the time but not 100% of the time.....? ANY help would be GREATLY appreciated, I'm slowly going mad trying to figure out what goes with what on these orders.

Thanks SOOOO much in advance!

Jen