run your shop as a business. Even if you're doing it just for fun, with the recession on, it's important not to lose money on each sale. The web of small business is such that with our sales we buy other things and move money around. If you lose money on each sale, you can't move any money, can you?
And BTW: it seems that school trips are fraught with peril. That said, we all have different beliefs, and yes, all beliefs come with a risk of not being business friendly. I curse a lot, and of course, most people wouldn't want to expose their kids to my blue streak, and many people want to tell their children about their own beliefs and not have some beliefs seemingly endorsed by the school, and others ignored.