Module for selling by weight and being able to adjust final weight from default weight

[nfarra]

Hi, 

We are building a PrestaShop site for selling our groceries products. Many of these are sold by weight (e.g. Cheese) so we want to set a default weight that will be displayed at the Site and that will be used to calculate the default price. Let's say cheese A has a default weight of 0.250 Kg and a price of 10 € / Kg. That would set a default price of 2.5 € per "unit" of 0.250 Kg. If a User buys this products he or see will see:

Cheese A: 0.250 Kg * 10 €/Kg = 2.5€. 

On the physical store we will do our best to cut the cheese as much as we can to match the 0.250 Kg but we will more than likely miss the weight ending up with a piece of something between 0.230 and 0.270 Kg. Let's say in this case we cut a piece of 0.240 Kg. At this step we would like to be able to "adjust" de original order to match this final weight. This way the user will instead see:

Cheese A 0.240 Kg * 10 € / Kg = 2.4 €

The final bill will be available just once all the weighted products have been issued and it should be the definitive one.

I know that payments methods (e.g. RedSys) allows these kind of adjustment by pre-authorization payment models.

To integrate this solution into our PrestaShop site we would need a double functionality:

- At the end of each order we should therefore be able to differentiate between original bill and the and the final bill.
- Be able to handle the pre-authorization payments by sending the correct parameters to the Virtual TPV / PayPal

Do you know if there is any module that allows to work this way?
How much would you estime that could cost if there is such module in the current market?

Thank you very much for your time!

Núria