No one in the PayPal community seems to have an answer to this, so I'm hoping e-e may come through for me,,, In PayPal's Website Payments Standard "Add to Cart" button creation form, there doesn't appear to be any way for the user to choose a quantity; the shopper can only add the item and THEN change the quantity, which isn't at all intuitive.
The obvious workaround is to add a drop-down with price/options (1 acme widget - $1, 2 acme widgets - $2, 3 acme widgets - $3, etc) -- but this is so clunky that it's not much of a substitute. And it means that if the shopper chooses anything other than 1 widget, they will only be able to change quantities in the cart in MULTIPLES of the number they chose originally.
Surely there's another way to do this? Points for urgency and apparent difficulty. Fingers crossed! Thanks folks.