On my site, existing customers can purchase other services on a discount. I have displayed two prices: for example £199 for existing customers and £249 for new customers.
Those who are existing customers, they will click and go the next page where they will see a text link that says:
Click to enter verification code & pay discounted price.
When they click the link, an HTML form will ask the user to enter a verification code. This is the code they have received in an email when they previously bought a package from us.
And if you are dealing with money: not even think of.