I'm an ex-clyde so I'm not bashing overweight people at all.
If it's not a mechanical defect or poor workmanship causing the problem, but the fact that you're that heavy that you are causing your wheels to go out of true, it's kind of ****** move to expect the LBS to keep fixing the problem at their expense.
I've dealt with some ****ty shops in my time, there are places I avoid like the plague. However, at some point, the responsibility of educating yourself on buying a proper wheel type for your weight/mileage/terrain/riding style falls on the purchaser not the seller.
Imagine how the bike shop feels about constantly having to give away free service because of buyer ignorance.
You can teach yourself how to properly tension and true a rim by watching Youtube videos or park tools also has some maintenance videos.