This is a limitation of our site but there's an easy workaround (sorry for the inconvenience!) The "builder" lets you add up to 10 items, so you can do one of two things:
1) add a single quantity of what you want (i.e., just one battery, or two if there is a minimum) and then add those to the cart. Once in the cart, increase the quantities to what you want and then click "update" at the bottom of the cart page.
2) add the items ala carte by going to their individual product pages and clicking 'add to cart'. To do this in the builder, just click the title of the product, it'll open a new tab with that product page - on that page, click 'add to cart'. You can further adjust quantities on the shopping cart page, then click "update" at the bottom of the cart page.
Contact us if you need anything at all!