What's the best way to set a minimum number of units per subscription plan? Specifically for use via a hosted payment page.

Example:

- Plan 'Standard' needs a minimum of 4 units before checking out

- Plan 'Professional' needs a minimum of 10 units before checking out

... etc etc

So when a customer gets through to the payment page, they are unable to check out by editing the quantity of units to lower than the minimum.

Hope that makes sense.

(I'm aware you can set a plan's unit quantity when creating a URL to the hosted payment page, but that doesn't prevent the user from editing the quantity before checking out).

Thanks!