It depends on the artist you're commissiong.
Here on DeviantArt they have "account points" which can be converted into real money. 100 points equals $1 USD, 200 points equals $2 USD and 1,000 points equals $10 USD etc. Some commisioners convert their money into points so they can commission artists who only accepted DeviantArt points.
Some artists however accept Paypal only so all you need to do is pay them via Paypal. There are other sites like Gumroad as well.
There's no one rule and you don't necessarily have to use DeviantArt and their system to commission an artist, it's all about what works for you and what works for the artist.
I hope this helps.