[BNM] Caculated amounts for online payment

[Stephen Carter]

We have a client who reads manuscripts, each manuscript is a different
length. Their pricing structure for manuscripts over 200 pages is £1.50 per
page for the first 300 pages and £1 per page thereafter.

My clients are happy for the their customers to calculate this themselves
but I am not sure if it is possible/secure for them to type in the amount
they wish to pay.

How can I resolve this?