I did that, just the once. A few weeks later I had a front tyre blow out on the M23 at 70 MPH at night. Fecking scary and very luckily there was no traffic around me when it happened.
Tyres, shock absorbers and brakes should be replaced in pairs ideally
No big deal if you don't change two tyres as long as they are the same brand on each axle I don't think
No doubt a mechanic poster will be along soon to confirm or refute I'm sure
I don't think that you are being ripped off...