YOu're not being petty at all. YOu did the job; they have to pay. You're not in business to give charity, are you? There are people who think that if they ignore you, you'll do just what you are doing now (weighing if it's worth it to collect), and that you'll eventually just go away. That sets a bad precedent, and also cuts into your bottom line.
I sometimes do collections as part of my job, and I've found that a steady stream of reminders (non-harassing) will usually do the trick. It seems that it's a pain to get a friendly phone call every day for a week: "Hi, it's me again. Just checking on the payment status of our invoice." I personally hate doing collections, because I hear the same BS excuses all the time, and I have other things to do besides babysitting customers...but collections have to be done sometimes if you want to ensure a steady cash flow.
In our business 99.999999% of our customers are repeats, so if one doesn't pay on time, I can revoke their credit terms and they will have to pay up front for everything. That's a pretty powerful persuasion tool.
We had a customer in the Dom Rep who tried to stiff us on $80 in freight, did the whole ignore bit, wouldn't return calls. So I just drafted them a friendly but firm fax which said that although there seems to have been some miscommunication surrounding the job, and over who was responsible for the freight (lets them save a little face if you don't outright call them a shitbag), the freight was still due, and if it wasn't paid, then all future jobs must be paid in advance, and the cost of the next job would include the old freight charge, plus interest, plus administrative fees for the time we spent trying to collect, and that I would also notify all of our business associates of this company's refusal to pay, which would make everyone shy away from them...we got a check via Fedex in three days.
Be persistent and firm, yet don't fly off the handle. What's keeping you from showing up at the guy's house with an invoice, a friendly smile, and politely asking him to write you a check on the spot?