Shopify Product Page Customisation

Hi,

I’m looking for some help customising our Shopify product pages.

Our website is wearthat.co.uk

At the moment, customers have to select one size at a time and add each item to the cart individually. This is causing friction and likely affecting conversions.

We want a size quantity selector on product pages so customers can:

Enter quantities for multiple sizes at once
(e.g. 5 Small, 5 Medium, 7 Large, 10 XL)
Then click one button to add everything to the cart
Instead of selecting one size at a time, we’d like a simple layout like:

Small [ 0 ]
Medium [ 0 ]
Large [ 0 ]
XL [ 0 ]

Customer enters quantities, then clicks:
Add to Cart / Start Order

This should add all selected sizes (with quantities) to the cart in one go.

Must work on mobile (most of our traffic is mobile)
Must work with our current Shopify theme
Must integrate with standard Shopify cart/checkout
Only sizes with quantities > 0 should be added
Keep the existing text box for customers to enter colours/notes
Please let me know:

If this is something you can do
Estimated cost
Timeline
Thanks,
Sara