I second everything everyone here has said about needing the Clavamox. Yes, it's good that you started it. Under the circumstances, I would give this vet your business. I was thinking --get a prescription from her and buy online... but given how you said she handled it, hey, let her have her markup on the pills. I"m surprised she didn't tell you that about the full course being critically important for anti-bios.
LovesMyCats offered you cash. Take it. I will kick in a little bit, too. Take $$$ from anyone who is willing to give it to you for your babies.
Here's how I see it. As an adult, you make decisions for yourself and live with the outcomes, good or bad. but the kitties are your dependents and "depend" on you to make the best decisions FOR their well-being. It is in Wilbur's VERY best interests to take a full course of ABs for his infection.
It's best that the line of dosing be unbroken... no skipped doses. So the first dose that would be 'missed' is Saturday morning. So you need the remainder of pills in hand by then for optimal results!
The only issue I see here is if Wilbur has a reaction to the drugs, which could happen ... and he will likely get the poops. A little canned pumpkin in his wet food will help with that -- about a level tablespoon. You can get canned, cooked UNSEASONED pumpkin at Walmart's grocery or any grocery store. (NOT pumpkin pie mix).