Changing price and adding a tax value to a hand coded cart / PayPal integration

Hi Ray. I hope you're doing ok man.

I've a quick question about the Sports Medical Certificates website (www.sportsmedicalcertificates.com) you'd assisted me with in the past. This website has quite a convoluted form but, essentially, passes a single value (65.00) to PayPal for the card processing bit.

The client is now VAT registered so this price needs to change to 62.50 and also add a 20% vat value (12.50). There was significant hand-coding on this, at the time, so I'm a bit terrified about changing anything universal so wanted to ask your advice on it.

I can open the eCart behaviour and I note that I could add the Tax value in there. That would be a start I guess.
Will that value automatically be passed to PayPal?

I've found the Add to Cart button and it has this line of code:

$ATC_itemPrice = floatval("65");// column binding

If I change this static value, I assume I won't need to change it anywhere else.
That, and adding the TAX value via the eCart server behaviour panel, and that should be it, right?

Would appreciate your advice and guidance. Thank you.
NJ