[AJackson]

Since this is just now coming up and there is no set policy I assume that it's not something that happens often.

Employees get into patterns regardless what they were told upon being hired and they view their current schedule as what they get paid for and will consider the extra hours as working for free. From their perspective the company is getting paid for the extra product (around here I even ahve to pay extra for Saturday delivery), why shouldn't they also benefit?

Obviously, though, you shouldn't be working all the extra shifts yourself. Doing things as you are doing them now is weak and you'll employees will never volunteer. If I were you I would post a memo reminding them what the expectations are and make a rotation list. If you're next on the list, it's your shift. If it's an issue then they can trade with someone.

As a manager who realizes that happy employees are better than unhappy employees I would come up with some sort of compensation for working these extra shifts. If a bonus isn't possible, then perhaps after working an extra shift they would be first in line to take off early on the next slow day.