Clients rarely doubt whether you can do the work. They doubt whether you'll still be around in six months — and every scrappy detail feeds that doubt.
Pay Links for the quick jobs, invoices for the ones that need one, and Price Quotes with payment terms that invoice automatically for the big ones. You connect your own payment method, so the money goes to your account.
Being one person is an advantage — the client deals directly with the person doing the work. Pretending to be a team fools nobody and gives that advantage away.