I have a simple policy for the "can you leave it this time.." ones.
I say "sure, but if I do, I won't be coming back ever, so its your choice"
I make an exception if there's a good reason of course, like if there's building work going on.
One woman said "oh, well there's no need to be like that". Don't know why, but that day it really got on my wick.
So I said "sorry but there is every need to be like that. I provide an excellent, regular and reliable service, not an every-now-and-then-when-you-feel-like-it service. I have a long list of people who are just waiting for a space to come available in my round, so I'm afraid I don't have time to messed around. You either want my service or you don't. You can't have it just some of the time, that's just not how I work."
She said "ok, I understand, yes you do provide a good service" and never dared to try the "not this time please" line again.
I did feel a bit bad about it after, because I was a bit harsh, but it does annoy me when customers think they can mess you about. I think a zero-tolerance policy is the only way really.