buyers.py has two new methods: updateBuyerPaid() and hasBuyerPaid().  The former sets the buyer_paid value from no to yes, and the latter returns true or false accordingly as buyer_paid is yes or no.
console.py has been updated to display the new Buyers table including the buyer_paid column.  Next to do is to change buyer_paid from no to yes when a receipt is printed and to overlay the PAID image onto the receipt.  This will require a query of buyer.hasBuyerPaid each time a receipt is previewed or printed in a summary.
buyers.buyer_paid is set to 'yes' when a receipt is printed for a buyer.  Next problem is to overlay 'PAID' on a receipt when it is printed.  This is going to complicate printing and previewing because I don't know a buyer number when I print or preview or a summary.  Thus, I may have to have separate printing methods for Portrain and Landscape according to whether it's a summary or for an individual buyer or set of buyers.
Added a new method printPortraiWithOverlay that will print receipts with PAID overlay when the buyer has paid.
Can print a receipt with PAID overlay.
Removed some print statement from printingmanager.py.
Can now check whether a buyer has anything in his cart before printing a receipt and marking the buyer as having paid.
Printing all receipts now only prints for those who have paid, whereas all carts are printed -- regardless of whether paid or not.  Should I only print carts for those who haven't paid?
I've realized that I can't overlay PAID over any of the summaries because the buyer number isn't known at the time I want to print (the whole print file has been built elsewhere, so printingmanager doesn't know the buyer number).  Thus far I can print the set of receipt pages for people who have paid.  I still need to make the compressed summary print only for those who have paid.  After that, there's the question of what happens when no one has paid.
Removed debugging print statements from printingservices.py.
Removed debugging print statements from printingservices.py.
I think that I've done all the new code to print or preview only shopping carts with some entries and receipts that have been paid.  What's to be done now is to make that actually happen and remove lots of print statements.
Right now I'm still printing all carts.  That requires some more thought.  It might still be good to show all shopping carts so that a buyer can be given paper to show that nothing was purchased.  BIG PROBLEM: If I cancel a purchase and a receipt has been issued, then a new receipt must be issued.  If the buyer had purchased only one item, then there's no receipt.  Best to change buyer_paid to 'no' after canceling the purchase and then issue the receipt again as a normal procedure.
All of the printing and previewing has been coordinated so that receipts are printed only when the buyer has paid.  For the moment I am continuing to print all carts because I think that a buyer would like to know that his/her cart is empty and there's nothing to pay for.  I'm committing here and to Github.