Well, it still is a risk to not receive a payment, if you let the customers pay themselves. Although, you could simply wait with working on their commission until they paid you, so you won´t do the work for nothing.

I do insist on being sent payment after having a sketch approved, it's one of my terms ;u;

I don´t see a problem with either way of being paid. I, for myself, never had problems when I was selling YCHs over my old account - and I never did the invoice thing. The only annoying thing were the transaction fees when I received my money in a different currency (like, when paypal switched 20 USD to Euro) but besides that? Never had problems.

You only have this limit as private person. In Euro it´s 2500€ in a year that you are "allowed" to receive. Everything above is considered as commercial and would require you to upgrade your paypal account to a business account, as from what I know.