Venting: people keep treating my pet sitting like a free service

I pet sit on the side because money's tight and it helps me cover bills. I love animals, I'm careful, and I try to run things by the book so nothing gets messed up.

What's driving me nuts lately is how some friends and acquaintances treat it like it is not a real responsibility. Because I'm already watching one pet, they assume I should just add theirs. Or they'll ask, "Can you swing by and do a quick check while you're in the area?" as if that takes no time, gas, or planning. One person even tried to drop off a pet with no carrier and no supplies because it would "just be for a night."

I get that people are stressed and money is tight. Still, I'm not a free daycare and I'm not comfortable taking on unknown pets at the last minute. It's not only about the money. Safety matters. I need a routine, vet contact, feeding instructions, and to know about any behavior issues before I say yes. I also have my own pets and I will not risk bringing home fleas, illness, or extra stress.

I hate feeling like the bad guy for having boundaries. I keep saying no politely, but it's starting to make me resentful.

How do you shut this down without turning it into a whole thing?