It kind of depends on the nature of the local store.

If it's a locally owned hiking/backpacking/mountaineering shop with high expertise, I'd patronize it, especially if it carries lightweight gear (not many do). You did utilize their skills and time, and that is worth the extra $30 (remember that Campmor's price doesn't include shipping), if what they have is what you want. It's well worth the extra money just to have the local store! Also note that the Campmor pack is undoubtedly a prior year's version on closeout so is probably not exactly the same pack that you tried on in the store (most manufacturers tweak their models frequently). If so, you may need to spend another $10 to send it back. About that time, the store's price won't look unreasonable. A small local store generally runs on a very tight budget for overhead and will suffer a loss from significant time spent with non-buying customers. In other words, if you spent an hour being fitted and then don't buy their pack, you have caused them to suffer an economic loss--akin to stealing. This would be fine if none of their merchandise met your needs, but in this case you're just trying to get the same pack cheaper.

If, on the other hand, the store is a branch of a national chain store (like REI, alias Really Expensive Items), I don't hesitate to walk out without buying anything even if they've spent a lot of time with me. I make it clear to the staff that what they have does not meet my needs, and why. As Phat says, they make their money from high-volume sales elsewhere (selling "sports" clothing and loading down unwitting beginners with super-heavy "traditional" gear while lightening their wallets). Unlike the locally-owned store, the chain store has budgeted plenty of overhead for time spent with non-buying customers, a miniscule amount compared with their total sales volume.