Hello, I would appreciate to know if it is possible to gain 2 t-shirts and how much amount the expenditures of shipping.
...oltretutto mi pare proprio che si dovrebbe usare "whether" e non "if".
L'unica accettabile mi pare quella di andrea.paiola.

Io cmq tradurrei:

Hi, I would like to know whether it's possible to buy two t-shits and how much is the shipping cost.