When leveling Enchanting (for free) with their mats, I never charge for anything.
And once my profession is maxed, again I never charge (still their mats).

I think I've had maybe a dozen people not tip.
Which is not a big deal, since I'm not asking for a tip/combine fee.

But not asking for tips, results in much higher gold per hour (from tips), then asking for any amount of a combine fee.
Back when the 15 Agi to gloves enchant was only from a drop in AQ, instead of a faction reward... I received a 100g tip for doing this enchant to a pair of level 19 rogues (their mats too).