Hey Robin-
Welcome to the Disney Parks Moms Panel. I love the Disney Dining Plan because we leave all the cooking and cleaning to Mickey and the Gang.
I have used an authorized Disney travel specialist many times. With a travel agent you may add a Disney Dining Plan as long as 1. It is through the travel specialist, 2. You have a reservation at a Walt Disney World Resort hotel and 3. it is added 72 hours before your resort check-in date. Please contact them to add a Disney Dining Plan to your Magic Your Way Vacation Package.
When you use a third party vendor like Priceline or Costco, they become your travel agent and they need to negotiate with Walt Disney World Resort on your behalf to add a Disney Dining Plan.
I hope this helps, Robin. Please
let me know if you have any other questions.
Enjoy making many Magical Memories!
~Jason ºOº