People who do comissions

I mostly use PayPal though I am looking into alternatives. 

Make sure they pay you first before you give them the full work, when showing progress crop.

Each person prices their work differently, some have set prices for different varying levels of their art. Sometimes I price my work by time or use set prices.

I mostly use PayPal for payment but if they're in the same country as me I prefer ETransfer.

Paypal can be easy bc you can just send them a link for payment and then transfer it over to your card.

As for pricing, my beginning prices when I just started were on the cheaper side, but I found doing comms for cheap was not worth it. It is very draining, and you don't get paid adequately for your time and effort. I would consider how long a piece takes to create, how you can seperate your works into different tiers (such as icons - coloured sketches - flat colours - full renders) with the higher tiers being more expensive, and I'd say the more comms you get take that as a sign to raise prices to match your demand.

Hope this makes sense lol, I'm no expert on this stuff, but I hope this helps :)