Add a charge

Hello, I am building a website to sell diesel truck parts, however im having a hard time adding an option to where they can choose to send in their old product which won't charge them, and another option to send it in later, which will charge them. I tried doing it on variants, but I'm not sure if I can sync the inventories with each other. Like if I only have 1 item in stock and someone buys it with the option to send in their old product, it will still have 1 as available for the other option of keeping their old product. Ill attach a link to a website that has this option. If you could help me achieve it, it would be great.

https://cncfab.us/products/96-03-7-3l-cnc-fab-stage-1-hpop-424017

In the bottom right it has an option to add a core charge or not, this is what I'm trying to achieve. Thank you