Tipping the seamstress?

I had my last alterations appointment last week with my seamstress who was incredibly wonderful and helpful turning my MIL's 80's gown into the dress of my dreams (not substantial difficult changes, but still). My mother paid for the alterations by check and I took my dress home.

I'm sitting down to write her a thank you note and I'd like to give her a tip in cash, even though I know it's not necessary. I was thinking giving her a $20 (10% of my alterations cost) but I'm worried that might be offensively small? I can't afford to tip her much more than that with the tight budgeting as we go into the final stretch of wedding planning and paying (I did not originally budget to tip on the alterations).
Do you think the $20 is fine? Should I skip tipping her all together and just write a heart felt note?