For most of us time is almost as valuable as money if not more valuable at times. I do my best to not make to many shopping trips where I am going out for just one item but when I do I make sure to call the store ahead of time. There are really two reasons for this.
The first one is more obvious in that if the store does not have the item or if it is a higher price than what I am willing to spend I just saved myself time and gas. The second reason is something I have come to conclude only recently. Most stores will gladly hold an item for you at their register or service counter if you call ahead to reserve it. I like to do this not so much because I'm worried it will sell out ( although another good reason). I do it more because then I can go right to where the item is being held with out the temptation of impulse add on items. How many times have you gone into a store to get just one thing and walked out with a ton more? Your ten dollar trip can quickly multiply when you are wandering the isles.