Maybe you are.
While base products, like TP or toothpaste, are more expensive on Amazon than in physical stores, the price difference isn't *that* much. To some people, an extra dollar or two is easily worth not having to worry about it at the store next time. If you tally up your yearly usage of toothpaste (say, if you're an insanely prolific tooth brusher, or have a family) to be a giant tube a month, that's $30 a year from Amazon as opposed to, say, $12 from a real store.
If you're well-organized and go to the store regularly, the $18 isn't worth it. Personally, I am not perfectly organized, and am insanely busy. That $18 difference is worth forgetting about it in the store a few times in a row and going without toothpaste for a week. Of course, it's not even an $18 difference: I probably go through two tubes a year, so it's a $3 difference. That's almost literally nothing.