The same private link your client has been uploading documents to is where they pay. There is nothing new for them to learn and nothing new to sign up for.

What they see

A payment block appears on their page: Payment due, the amount, your firm's name, and a Pay now button. It is marked Secure payment by Stripe, because that is who handles the card.

When it goes through they see Payment received and a thank you, immediately. They can download the invoice for their records at any point.

Card details never touch Vylan

Payment is handled by Stripe end to end. Vylan is told whether a payment succeeded. It never sees or stores your client's card.

When a payment fails

Cards get declined. Vylan says so plainly and offers Try again rather than leaving your client staring at an error. If you are not set up to take payments yet, they are told to contact you to arrange it instead.