The way I address this is to get them in the door, and then say "It was half hour/one hour/two hours you wanted, right? That's X amount" and then just hold my hand out for the cash. Count the cash in front of them - I've been shorted before, but equally I've been given extra and I've always said "My hourly rate is 120, did you mean to give me 140?" I know some girls will say just take it but I'd rather they give me a tip after. If it's short just say "This is only 60 darling, my half hour rate is 70". If they say they haven't got any more then I tell them I'll give them my 15 minute rate for 50 but it doesn't include full sex. Miraculously they manage to find the extra money!
If they stand looking at you like a stunned lamb, just say "Let's get the business out the way first, so we can get to the fun stuff! It's ?120" and until the money is in your hand, you do nothing.
Do get yourself to the clinic and then keep going once a month or so. You can usually get free condoms there if you need them. Most HCPs I have dealt with there have been very professional and supportive.