if its completely free, what is their revenue model?
Don't ask questions that can be answered with a very simple
google search. Quite redundant.
You can also use BitPay or integrate Electrum on a website of yours. BitPay is free up to certain point and Electrum is completely free.
In case that OP (or anyone else) is interested in the details of the free tier, here it is:
30 Transactions / Month
Up to $1,000 Daily
1% applied to additional transactions