I am a shopify partner, I am trying to get around the the variant limits of shopify by coding an app and also because my client has a complicated formula for calculating the price. I don't have a problem with the part that adds the information for my variants to the order, however when I want to modify the price based on the user selection, it doesn't seem an easy task to do.
I came across couple of apps which does the same thing by adding a new product or variation which is not optimal for my use case, is there any app that does the job without adding extra information to the database, if so, how?
Unfortunately, Shopify doesn't give us a lot of options to edit prices of items dynamically. Here are the two options that I know of that will allow you to adjust the price of a product directly - however, both have limitations.
If you're working with a Shopify Plus merchant, you can use the Shopify Script Editor to dynamically adjust the prices of products, provided that the direction you adjust is down. You cannot increase the price of a product using the script editor - so for your use case, the list price would have to be the most expensive possible price that the item sells for, which you would then discount appropriately using the app.
Using an app, you can use Shopify's Draft Order API to create an order with custom discounts and/or create completely custom items that are independent of the products set up in the product database.
The basic flow for this is that when the customer clicks 'checkout' you halt the normal navigation, send the cart contents to your app, create a draft order with the appropriate pricing, then supply the front-end code with the draft-order checkout/invoice URL so that you can send the customer there instead of the normal checkout. This has several limitations, however, including that prices of existing products can only be discounted, not increased, and the fact that Shopify will not allow a customer to use discount codes on a draft order invoice - once an order is set up this way, Shopify's assumption is that all prices are final.