Product price based on its measurements | Payment | File Upload

Site URL: https://www.signsbase.co.uk/

Hi All,

We just recently launched our business website on Squarespace offering promotional print and signage products and we are considering upgrading to ecommerce. I was wondering are there any extensions or plugins where:
1) The price will be calculated based on the width and height of the print that has been indiceted by the customer (for example 1250mm x 560mmh) - we can obviously sell pre-set sizes such as A4, A5, etc but we want our customers to be able to provide us with a custom size they require.
2) Is there a plugin where I can accept a file upload at the checkout?
3) Is there an option where customers can select "pay on invoice"?

Many thanks for all your help in advance.