Recurring Subscriptions for $0 Item

I want to have a product in my store for recurring donations. The base product should be "zero", then they should be able to choose from a drop-down list, a variety of options ($10/month, $20/month, etc.). I set up recurring payments in accordance with this idea. However, when they go to checkout, the item price is still zero, when the subscription price should be added to the base price. Does anyone have a fix for this? An extension? Anything?

Here are a couple of screenshots to help demonstrate the issue:

Well, this would probably require a Pro, to solve it, you should better have
placed it in the commercial Section. But some Extensions possibly exist for
this, as it looks.
Good Luck
Ernie
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Product Subscriptions - Recurring Payments
https://www.opencart.com/index.php?rout ... on_id=9890
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Front End Recurring Product Subscription
https://www.opencart.com/index.php?rout ... n_id=30725
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Anybody who finds this thread in the future would probably benefit from knowing how to fix this. In Opencart 2.3.0.2, you must edit this file:
system/library/cart/cart.php

Replace:
With this: Be aware that you need to have a trial period if you do this, else the customer will be double charged (once for the price of the product, once for the subscription).

Thanks!
This tip was indeed extremely helpful for me where the selling price depends on the chosen recurring profile.
So I changed system/library/cart/cart.php to: