According to Shippo, this error returns when there are more than 30 items added to a customs declaration. The USPS supports up to 30 items in a shipment. If you group same items together (by simply adding up the quantity) and keep the total under 30 items, you should be able to create the label.

While Shippo's suggestion allows you to create a shipping label, you may also want to print a Commercial Invoice / Customs Declaration Form that contains a full list of items in the shipment.