How can I modify a line item price or subtotal with Ajax?

Technical Q&A
1

I want to modify a line item price adding a variable charge depending on the product with Ajax temporarily and pass it to my checkout and be available to charge it.

I try modifying directly, changing the price like this:

const productData = {
      id: variant_id,
      quantity: qty,
      price:newCharge,
      properties: deliveryDate
      ? {
        _handle: handle,
        ...customProperties,
          }
        : { _handle: handle, ...customProperties },
    };

    await $.post("/cart/add.js", productData, null, "json");

It seems not to replace directly the price, I also look to the documentation to take in deep about the properties supports, but I didn’t see anything that would help me about.

But if there’s no way to modify the line item price, I was trying to modify directly the subtotal adding the charge that I need. So that’s it basically

Any suggestion or opinion would be good to receive it