You can sell supplies to get money.

With PCC, this is rarely needed because PCC automatically sells supplies when needed for buying something, and you can sell supplies during cargo transfer, but sometimes can still be useful.

The "Sell Supplies" dialog will ask you how many supplies you want to sell. It will sell the supplies when you confirm with Enter. You can also enter the number of supplies you want to keep and confirm with A.

Reference

Keys:
S from planet screen.