My client wants the following functionality on their website:
1 - Person books a course via phone or email.
2 - On the website, there is a "Pay" button where the user can pay for their booked course.
3 - User is sent to SagePay where they can enter the price of the course they booked, along with payment details, and make payment.
Is this possible? I've been scouring the docs, but from what I've found you have to send the amount through to Sagepay.
You need to send the amount with the transaction registration post. You could ask for it before building the post.