I've said it before and I'll say it again. Training collars are not the greatest evil known to dogdom. They are tools that can be used correctly and safely or incorrectly with disastrous results. (Prongs in particular must be fitted very carefully.) A good trainer should, before asking you to put any training collar on your dog, instruct you to put one on yourself so you'll know what it does, what it feels like, and what level of force to put behind it. Alternatively, you could try one of the no-pulling harnesses. There are also groups on lj: dogtrainers (tag list here) and dogsintraining.

When you were pushing the cart at Petsmart did you give him less slack in the leash than usual and keep him in close to you and the cart thus making it more difficult for him to pull? You might want to work on getting him to heel very close to you and putting that special heel on command. On ordinary walks I don't care if my dogs heel slightly in front, slightly behind, or directly beside of me as long as they don't pull, but they "close up" at bridges, traffic intersections, or when we pass other people.