100 phone calls will get as good a result as 1,000 mailers. In a small town there is a chance that people who receive different offers may compare notes and wonder what's going on. Be honest and tell them you are testing some advertising they will understand. Always honor whichever offer the customer wants. Also always provide the customer with the item at the lowest price being tested.
Tests can be expensive.
Only use them to test the important things. The test may reveal some negatives about your business. Learn from these numbers and make the necessary corrections. That's the next item I'll discuss. In order to know if your advertising is really pulling customers you need a way to track the ads.