Portal users (by themselves) do not count toward the number of 'licensed' users.  BUT, they cannot also be regular users (like an employee).  if you make an employee (a valid user for licensing purposes) a portal user they are still an employee (and still a valid user for licensing purposes).

The only way a user will not be a paying user is if they have the portal checkbox checked and if the content of the Portal's group of access rights groups has been validated by Odoo (especially for write accesses).

(Italicized text is a direct quote from Odoo the last time I asked)

What 'validated' by Odoo means is that they can only write to the database in ways which makes reasonable sense for portal users. Changing the delivery date of an order would likely be a reasonable change a portal user would be allowed to make, but you will have to check with Odoo for specific guidance.