This is a link to the Federal Trade Commission's website discussing its rules regarding internet orders. Rules include when refunds must be made, including when shipment is delayed or a buyer cancels.
The Act is titled "Mail or Telephone Order Merchandisse Rule"; but, it applies to internet sales (as you'll see in the discussion--and in recent cases in which it was applied).
A Business Guide to the Federal Trade Commission’s MAIL OR TELEPHONE ORDER MERCHANDISE