paying for bridesmaids hair?

there are a few of my BMs that went above and beyond throughout this wedding process, so i was thinking about paying for some of them to get their hair done. i wanted to make a coupon to give them (which i will be inserting in their TY cards for my bridal shower), but

1) how should i put that their hair will be paid for, but they should pay for gratuity? (or should i pay the gratuity too? but thats $80 plus tip for like 5 girls... which isnt even my whole bp.. i have like 8+.. and im already overbudget lol)

2) how do i let them use it the day of without having the other girls see (i dont want them to get jealous, but it's not like they helped as much as the girls im giving the coupon to)

Maybe don't put it in with the TY card. I would pick a separate time to give it to them and just let them know how special they are and that you wanted to do this extra thing for them. If you can get all five girls together at the end of the RD (or at the time you were planning on giving the gifts) then do it then. Or just maybe whenever you can get each of them alone. HTH!

ETA: maybe give them gift certificates from the salon and tell them that they just have to cover the tip.