personally I would strongly suggest Guerrilla testing for two main reasons:
1) the user is in a neutral environment so she/he would not be intimidated by the "formal test environment", plus she/he would not be hired for the test, so would not try to please the tester or try to demonstrate something. The gift usually is given at the end, to thank for the time.
2) Guerrilla testing is a cheap and effective way to find the major issue in the interface/system.
It's not the definitive solution but it's really helpful and could give surprising and inspiring results.
Of course it has a lot of cons, it's difficult to find the correct target, even if you'll perform it in the same store of which you're designing the webstore.
The tester should be careful in approaching people in the right way, usually is simpler in a places in which people is sat (as a cafe or similar) and is not going or doing something.
It Is important to choose the right time in the day and the right day in the week to try to find the correct target, it is important to chat a bit with them to build a basic relationship, it's important to introduce them into the task without influence their behaviours because they could be totally "illitterate" in that particular system/device, it is important to find the right gift because it should look like a thank and not a payment (e.g. typically in a cafe the tester offer a coffe and a pastry or a breakfast, in a shop a 10% discount giftcard and so on).
Moreover it's a bit more complicated to analyse the results, the observer should be trained, usually you can't record the users while they perform the tasks. And the facial expressions are a greater feedback than the words.
That's in short (and I'm not sure as clear I've been) anyway I strongly suggest you to try.