I didn't have much problem coming up with a money tip. For me it's simple – get all the money up front. I never send out a letter unless the money is in my bank account.
So that customers know what to expect, I have made it clear on my site that letters are to be paid before they are sent out, and the latest date they need to be paid. There were a couple of customers who wanted to pay after the letters were sent out, and I just told them that the letters would be sent out when I had received payment. They ended up going somewhere else!
I decided on prepayment because I didn't want to be wasting my time following up on a $15 payment.
To make it easy for customers, my sign up page redirects people to a payment page where they can pay with PayPal or direct deposit. This way they can pay when they place their order. This way I don't have to chase them for their money and I have the money in my account to pay for envelopes, stamps and everything else I need to prepare and post letters.
I know this won't work for everyone, but it works well for me.