This 8.40 shipping charge is via USPS Priority Mail Flat-Rate Padded Envelope (9-1/2" X 12-1/2") and includes the package, postage, $50 insurance and tracking. US addresses only and usually arrives in 1 to 3 business days.
It might be possible to stuff 3 shirts in this envelope, but it hasn't been tried yet! PINS CAN BE INCLUDED WITH T-SHIRTS.
Regarding Cups, they can be sent in this envelope alone or with Shirts & Pins. However, there is the chance that the Cups could crack if the package was subjected to a crushing force and the buyer accepts that risk with this package.
Shipping for orders of more than 3 shirts will have to be via Priority Mail Medium Flat Rate Box at $15.05 -- See that item to use that service (also, this would be the safest way to ship Cups).