Pricing by Weight

I have a client (a local farm) looking to set-up e-commerce on Squarespace. They sell/price products based on weight. 

Does anyone have any ideas on how to solve for this with Squarespace/Stripe?

Perhaps setting up a fixed cost that acts as a 'hold' price, that can be adjusted later when the produce/meat is picked up in person on a POS?

Any help is appreciated, thanks!