I would say the lack of a remove button is more likely to affect sales. I've quite often, with impulsive purchases, just given up if I face frustrating order processes.
If I've added 10 of something, but changed my mind and chosen a different model I'm unlikely to sit there pressing the minus buttons 10 times in a row.
I'm more likely to just close the site. I can probably buy those 10 things another day.
People aren't going to go through with purchases because it's difficult to change their mind, they're going to go through with purchases because it's easy. Once they've decided "Maybe I don't need this" a frustrating interface isn't going to swing them round to the "Okay, I need this!"
Imagine if you were in a store and you were browsing, then decided you didn't want an item - if the salesperson was to prevent you putting it down, would that make you more likely to go through with the purchase, or another one? (Incidentally, I've had people do this to me in Marrakech, it's incredibly frustrating).
A text box you can directly set "0" in would help, but I don't even want to fiddle around with a text box when there should just be a little delete icon.