Are you saving much, doing it that way?
I'm a COSTCO member, and can get Michelin 195/50 R15 V (82) PILOT SPORT 3:
£65.98 each fitted (£57.58 + £8.40 new valve, balance, nitrogen fill). Current offer runs out on 15th July.
4 tyres = £263.92.
In addition, once the 4 tyres are fitted, I would receive £25 credit vouchers for in-house purchases, or, if 2 tyres, £10.
So 4 tyres would work out at £238.92 (£59.73 each) net.
If you're not currently a member, Annual Membership Fees are:
Individual - £33.60
Trade Member - £26.40