1) Go to the part's page (
http://www.shmax.com/part_details/5867/thruster_gun_left in this case)
2) Click the "Modify (part)" link in the upper-right
3) Scroll to the bottom of the page and click "Link to a Product"
4) Find the product that you want to link the part to. You can browse or search for the product, or enter the product's ID number. The product's ID number is located in the product page's URL (shmax.com/product_details/
1752/ravage_and_rumble).
5) Once you've selected a product, it appears at the bottom of the page. Enter the quantity of the part in the field provided, then click the "Submit Part" button.
6) Profit!