Your first sale, first gig, first commission: someone had to ask for money first. Sending the invoice is brave too.

Doing the work is the comfortable part, because nobody has to put a number on you while you are doing it. Asking for money states a value and invites a reaction to it, and that reaction feels personal in a way that criticism of the work does not. It is the reason so many people do good work free for far too long.

Send it within a day of finishing, with the figure stated plainly and nothing apologetic around it. Then agree the price before you start the next one.