Error with variable payments!

“Error! Invalid payment amount!”

I’m getting this awful error with variable payments, the calculations used to work fine (sometimes) but now just getting this constant error.

It’s really a simple calculation. Choose between 2 options, with different prices. Discount is only visible/active if the correct code is typed in input field.

Calculation:
(selected-option)-(discount)

Comes out with the correct total every time, goes through to paypal totally fine, all good until you hit “pay”.

I’ve tried changing the format of the dates, I’ve tried changing the rounding and removing decimal places. Nothing works so far.

I’ve even rolled back to about 10 different versions, by the time it starts to work okay I’ve lost other features.

Please help!