Add Ecommerce Store + Square Payments to Existing Custom Website

PROJECT UPDATE:
Existing website has been provisionally identified as Laravel/PHP 8.2 with Bootstrap 5. Initial store will consist of one Blind Date With a Book product with seven stock-controlled category options. Guest checkout, Square payments, flat-rate Australian shipping, configurable preference questionnaire and website-based product/order management required. Please provide a fixed total project cost and timeframe.

I own a small Australian book and café brand, The Cozy Quill, and currently have an existing custom-built website hosted through VentraIP.

I would like to add a small ecommerce section to the existing website so I can begin selling my Blind Date With a Book products online.

The existing website and branding are already complete. I am not looking for a new website or redesign.

I need:

A Shop page integrated into my existing website
Individual product/category listings with images, descriptions and pricing
Shopping cart functionality
Checkout
Integration with my existing Square account for payments
Australian shipping/postage options at checkout
Ability to manage products, pricing and stock myself after completion
Order confirmation emails
Mobile-friendly checkout
Basic testing before launch

Ideally, I would also like the option for customers to answer a few questions when ordering, such as preferred romance category, spice level and books/authors they already own.

My existing website is:
thecozyquill.au

Please include in your proposal:

Your total fixed price for the project
Whether any additional plugins/software or ongoing subscriptions would be required
Estimated completion timeframe
Examples of ecommerce websites you have previously built
Whether you have experience integrating Square payments
What platform/framework you propose using with my existing website

Please do not propose rebuilding the entire website. I am looking to add ecommerce functionality to the existing site.

I am currently gathering quotes and will select a freelancer once I have compared the available options.