Yes, correct.
You send me £90 as a totally separate transaction through PayPal - not in any way linked to the t-shirt.
Then, I tell you an exact time an day when I will change the 'buy it now price' to £9 + the shipping cost (GSP). You then 'buy it now' for £9. I am making the assumption that you will pay any tax on the value of the official price for the t-shirt i.e £9. so if it's 10% import tax, then you pay 90pence - but you need to check and confirm this. The £90 would be outside of this as a totally separate transaction and not connected in anyway (like you give me a gift).
In total you will have sent me £90 + £9 + GSP postage. So you pay the same total amount, but you should not pay much if any import tax (but you need to confirm).
Let me know.