Adding a Custom Field for Can Deposit on Zoho Invoices

I'm currently facing an issue with Zoho Invoices and I'm seeking your assistance.

Here's what I'm looking to achieve:

I need to incorporate a custom field to account for can deposits on my invoices. These deposits are additional charges added to beverage cans and are refunded to the customer when they return the empty can. Can deposits are not taxable.

I believe this can be done using custom fields and formulas. I'm thinking of creating a formula that calculates the deposit only if the product has an attribute "is_can_deposit" or something similar. However, I'm not sure how to add this calculated value to the "Amount" column since it seems to be locked.

Another option could be to automatically add a second row called "Deposit" whenever a product with a deposit is added, but I have no idea how to go about this.

If the solutions I mentioned are relevant and can be implemented, I would greatly appreciate assistance in completing the missing part of the process.

Alternatively, if there's a more straightforward way to implement this feature, please share your insights. Any guidance or instructions would be immensely helpful.

Thank you in advance for any assistance or suggestions provided!