Custom calculator

We're working on an online store that sells tiles in Shopify. They are delivered on pallets with a specific number of square meters. We'd like to provide customers with the option to automatically calculate the square meters per pallet. Additionally, customers should be able to choose a different square meter quantity, but in such cases, the pallet would need to be repackaged, and the customer would be required to pay a handling fee. Does anyone have a good recommendation for an app that can facilitate this?