Many software companies outsource the actual ecommerce component used
when customers purchase their own software, allowing customers to pay
for the software then to instantly download a paid version. (Or,
download a trial version, then pay to convert to paid instantly at any
time during trial.)

1) How big is this industry in annual revenues? Data a couple years
old is ok.

2) Who are the top five providers? E.g., http://www.digitalriver.com
is one such provider.

3) What are the most common functions provided?

Thanks.