If HF is $70, but you give 4 $20's when you come in, that's $80. In some places, you won't get your $10 back unless you ask. While I doubt Ruby would try to palm the $10, if she laid it on the counter and it wasn't collected before or after the tip, then she gets $80.
Ladies are adorable, in different ways. Unless you're an @$$, you really cannot go wrong here.