Thanks everyone for your answers.
Ryan, I can't commit now because I'm still considering Shopify vs Magento.
If you build this app I would be interested in hearing about it, please do post here should it be the case. Honestly there is a significant amount of request for this feature in the forums, I'm sure you'll be able to find clients for it.
https://ecommerce.shopify.com/c/shopify-discussion/t/in-store-pickup-option-w-pay-on-pickup-66777
https://ecommerce.shopify.com/c/payments-shipping-fulfilment/t/pickup-order-and-deliveries-112135
https://ecommerce.shopify.com/c/shopify-discussion/t/valid-pickup-option-5529
https://ecommerce.shopify.com/c/shopify-discussion/t/buy-online-pickup-in-store-143931
etc