It's a gray area, but if you're honest and keep good records you can make it pretty black and white. Proving intent is difficult so the IRS will give you some leeway and assume you're being honest, up to a point. If it becomes a track record that you're driving 90 miles and bringing home $30 of inventory regularly, they won't be amused.
I like to stay honest - if the trip is intended for business and pleasure 50/50, then I split the costs, including mileage, 50/50 and document why that's true. If it's 90/10 in favor of pleasure, I take only the very specific additional expenses for the 10 percent business (the mileage from my aunt's house to the nearest Target, rather than from my home to my aunt's nearest Target), and ditto if it's 90/10 in favor the business, I don't count the 10% of mileage to get from the four Kmart blowouts to meet my friend for lunch, but I count the other 90%.
Documenting how you do things and showing that when things should go in the IRS' favor they do, will keep you in good odor in the event of audit. When every mile on your car was, "Well, I was thinking about LEGO part of the time so it's a business expense", you're not going to find the IRS a sympathetic audience and they're going to move on to the orifice probe portion of the audit and spend a lot of time there.