There is never a direct fee for the parish priest, deacon or lay minister. Given the financial challenges many of our parishes experience, a generous donation at the time of funeral is very much appreciated, even being mindful of services provided to the deceased throughout the years as a parishioner.
And the price of a first class (1) ounce letter sent through the United States Post Office remains at $0.49 for letters mailed within the United States. The fee will continue to apply and extend for some indeterminate period of time.
According to the US Post Office postage calculator (link attached to this answer) the current cost for the most basic postage, first class letter to from the US to the UK, is 94 cents. If you don't want to go to the post office to get the exact postage you need, three first class stamps would do it. You'd be overpaying, though.