A starting businessman should NOT be apprehensive about sending out invoices. There is no shame in asking for someone to pay for what they have purchased.

REMEMBER: you're doing them a favor by not forcing them to pay up front. A 30-day same as cash policy is worth {cost of capital}/12

If you think reminding the customer about their bill is "begging", "only a business necessity", or whatever, you are wrong.

Ask yourself: when was the last time you bought something and thought the store was begging by asking for payment. Safeway is not greedy, begging or anything of the sort when they ask me to pay for a loaf of bread. And you are not being greedy by asking for payment for your work.