Customer wants to add to a preexisting order

Backstory: We run a local online greenhouse. Customers can order plants now and we will deliver them locally to their door when it is safe to plant outside. Customers can also pick to pick up at the farm.

Two part question:

Customer wants to add more plants to her preexisting order. How can I do that?  If that is not possible I could have her pick on-farm pick up and just staple her new order to old one. However I have a $30 minimum order (which is for farm to door delivery) Is there a way to make a minimum order less if some one picks on-farm pick up?