Well you could use Cartloom for that, there’s a 30 day free plan which covers all of its features (it is about 10$/months afterwards). It allows you to set up a product (so you could name it custom sandwich f.e.) with so called “Options” where you can enter the customizations. The customer can select an option or even multiple options at once and finalize the order. You can also make these options paid, so when the customers adds onions the price would raise from 5$ to 6$.
Once the order has been placed (it also handles payment via CC or PayPal if you want) you’ll get an E-Mail with the exact order details.
And of course this product can be put into a RapidWeaver site.