Last week I had an awkward interaction at the Torchy's Tacos in Sugar Land. I was picking up dinner on the way home from work and ordered to-go via the website. The ordering process defaults to a 15% tip, but I didn't realize you're supposed to tip that much on to-go orders so I changed it to 10%. When I walked inside to pick up and gave my name, the cashier asked if there was a problem with my order? I said I didn't think so but I was just picking it up now so honestly had no idea. Cashier then asked why I was only tipping 10%, at this point a manager noticed the conversation and joined in. Manager said that I need to tip at least 20% on to go orders as that's how the staff get paid, and added that if I wasn't going to tip adequately then I should find another restaurant. I felt bad about the whole thing but I didn't realize we're supposed to tip that much on to-go orders as we do for full service.

Was I in the wrong here? How much do you tip in this situation?