Merriam-Webster's online dictionary
tip : a gift or a sum of money tendered for a service performed or anticipated : gratuity
gratuity : something given voluntarily or beyond obligation usually for some service; especially : tip
Compact Oxford English Dictionary of Current English
tip : noun 1
a small sum of money given as a reward for services rendered
Princeton Dictionary
(v) tip, fee, bung (give a tip or gratuity to in return for a service, beyond the compensation agreed on)
Since $80 for a Thai massage is neither a small sum of money, nor given voluntarily beyond obligation or the compensation agreed on, it is not a tip.
The amount of money you agree with your MP to pay her for the chosen option is her salary, since the vast majority of them don't get a single $ from the entrance fee.
On top of the mandatory option price, anyone is free to give a tip (usually $10 or $20), which I personally almost never did, because I strongly suspect MPs to earn 2 or 3 times my salary.