If you have filed with us previously, but an amendment is needed, the amount that you have paid before cannot be deducted from the amendment fee amount. Also, there are usually no discount codes for the Amended returns on Sprintax.
If you do NOT need an amendment yet you have somehow entered the amended return module and are asked to pay the fees to have an amended return generated, please log out of your Sprintax account then log back in and choose the relevant tax year then uncheck 'I want to correct my tax return I already sent to the IRS' so that you'd be able to log into the normal tax filing module.