I'm trying to use the pay by grin option but it seems grin has moved on from sending by file or something? the command recommended on the payment page is deprecated, and it won't accept slate code either?
I'm not sure if your problem has anything to do with the network upgrade that happened in January this year - are you sure you have a compatible wallet?
The fourth network upgrade is expected to happen in mid-January 2021. Compatible versions of grin node and grin-wallet will be versioned 5.0.0 or greater. grin-miner will be unaffected. The first releases of these are scheduled for December.
The copper membership is supposed to be like $20 and if I pay in bitcoin then the fee will be 3 times as much
Not really, at the moment you can send a standard transaction for only about
$3 and get a confirmation in the next block - or pay half less for a confirmation in the next 6 blocks. You are probably relying on your wallet which always shows you the highest possible fee, or simply has a bad algorithm that calculates the fee based on the current state of mempool.