Adam:
Several things that people forget when they purchase something is cost per mile of the vehicle they use to get to the store, and the value of their time.
It has been awhile since I checked on what the average cost per mile is for using a motor vehicle. The vehicle rental companies and Consumers Union which publishs Consumer Reports are two good sources for cost per mile to operate a motor vehicle. The last time I looked, which was several years ago, it was about $.42 per mile. So the next time you go to buy something "locally", multiply $.42 per mile times the miles you drive and add that to the cost of what you purchased, or find the current amount if you can find it at CR or from the auto rental companies.
The other thing to consider is the amount of money you make per hour on your job or hobby. Use that figure based on the amount of time it took you to go buy the item "locally" and add that to the cost of the item.
Chances are those two expenses will probably set you back more than S/H through the mail order route.
One other thing is that the time you save can be used to do something else constuctive and/or profitable and deduct the money you made from the cost of the item and you may have actually bought the item for free.
When I did private HVAC-R contracting, I charged $42.00 per hour for labor, and a $35.00 trip charge. Because most people wanted a bottom line price, I always fudged the estimate in case I ran into a problem which I would fix and eat the cost instead of telling people I needed more money for the job. In this manner I averaged about $60.00 per hour worked and kept my customers happy because I always did the job at the quoted price. This being the case, I had no problem giving a helper or friend $10.00 or $20.00 to pick up parts or do other errands for me. They were happy to get easy money for easy work and I smelled like a rose while being able to make more money by doing the work that paid me the high rate.
Bountyhunter