Don't just keep giving in until you get overwhelmed with suppressed crossness/frustration and snap at them. Much better to set your boundaries in an assertive, friendly way.
So, when they say, 'when you've got time can you take me shopping to fill my freezer', you say, 'that may be a long time, you'll probably be better doing an online order'
Is your DH supportive? Why has he let you get into the position of giving them lifts? He should be saying to them that it's been very nice of you, but they shouldn't ask any more as you have a lot on your plate (and will do for the next 20 years!) When they come round in the evening, your DH should say early on that he'll walk them to the bus stop when they go, or something similar, that precludes them hanging on hopefully for a lift.
I am a non-driver (medical reasons) myself, so I sympathise with them for the difficulties our car-oriented society presents. But I'd hate to have someone offering me lifts who is privately resenting it, and would much rather they didn't.
If they want a lift for one-off type reasons - e.g. a medical appointment in a difficult location, how about making it a reciprocal-type situation. For example: 'Yes, I might be able to switch things around to do that. Could you help me out by looking after the baby on Wednesday evening while I do xxxx (or whatever is actually helpful for you, obvs'.
Sometimes other mums are v. helpful to me by taking one of my DC to a party or similar that I couldn't manage without a car. I always make sure I reciprocate by doing something different which IS within my power, and helps them out.