I found this on the USPS site: SFS Claims Information
Any claims for a Stamp Fulfillment Services (SFS) order that was paid for but not received within thirty (30) days must include one (1) of the following: (1) A copy of both sides of the cancelled check with the endorsement date on the back, OR a photocopy of the credit card bill along with your customer identification number.
Any claims on missing orders, or any request for exchanges of damaged items, should be submitted to:
STAMP FULFILLMENT SERVICES
P.O. BOX 219424
KANSAS CITY, MO 64121-9424