H.L.
I honestly cannot imagine someone--anyone--calling and expecting me to drop what I'm doing to leave my house and help with something that is not directly related to an immediate health crisis. That said, you likely would not say to them what I would say: "No, I can't leave right now. This is when I can help with that."
You don't need to try to change them. You need to change how you respond to them (which you have) and how you allow your refusal to make you feel. For one, stop explaining to them what you are doing. That only opens the door for them to criticize it and argue with you about whether or not it's more important than what they need, which will never be the case. When they call and ask if you're busy, say yes, and ask what they want. Take control of the conversation. Here's what that looks like:
--Hello?
--Hey...you busy?
--Yep, what's up?
--I need you to come over and pick up this rug, so I can clean it.
--I can't do it today. How soon do you need it lifted?
--Right now.
--Okay, well I can't get to it now. When is your deadline for having it lifted?
--Friday.
--Okay, let me see what my week looks like, and I'll let you know when I can get over there.
Stay firm, and don't talk like you're thinking it over. As soon as they start "getting mad" or once someone says, "NEVER MIND!", you say, "Okay, we'll talk to you later." And then you high five each other for sticking to your guns and go back to hangin' with your peeps.
Don't try to change them. Fix your boundaries. Congratulations on your progress.