Twilio, either formally or informally, sent me a t-shirt twice when I found a bug. For me as a programmer, a bug on the 3rd party partner side generally means I spent some amount of my time figuring out that the bug wasn't something I was generating, but rather my vendor. Programmer time has a pretty real value to it, which no one likes wasting.
When I reported the bug and the support person offered to send me a t-shirt, I forgot about the time I spent on the bug and went "Neat! A t-shirt!". Now, when I look at the t-shirt, I couldn't tell you the specifics on the troubleshooting or how it ruined my afternoon 10 months ago. But, I can tell you that it felt like the vendor actually responded to my concerns. Some vendors haven't, and then I do the natural thing and spew vitriol about them across the internet and to my colleagues.
If I didn't like a vendor and they were replaceable, the second that I hit a bug that I felt the need to report, I would just not use that vendor anymore. Anyone who files a bug report has some amount of loyalty to a company that is not worth upsetting.