I want to apologize for placing this blame squarely on ShipStation. It seems this is a field formatting issue. I am able to enter a decimal value if I enter the decimal first and then manipulate the value using the left arrow and shift keys. Certainly not ideal, but this problem is unique to Safari on macOS (currently). I hope that ShipStation is able to fix this issue, however, because I have no issue entering a decimal value using the "old" ShipStation interface.