Could you please advice what shippment (couriers) operators are integrated with Shopify in Poland?
Because I can't see any - neither in those integrated nor in shopify apps 😞
At this moment most of the merchants in Poland use Baselinker for couriers integrations with Shopify.
If you would like to let your customers choose Paczkomaty InPost in checkout, there is a Shopify app for that: https://apps.shopify.com/paczkomaty-inpost (it integrates with Baselinker as well)
Maybe you can try to use Outvio? 😉 I have been using it for a while and I am very pleased with the results. I integrated this software with my online shop and can sell much faster and easier in Poland. Outvio has an integration with the main Polish couriers: DPD, DHL, UPS and InPost. Honestly, Outvio has saved me a ton of my time, money and energy (automatic returns, shipping labels printing, faster fulfilment ...)
You can go for Geodis or XPO for shipping from Poland. And to integrate with both of them & other top shipping carriers across the globe you can check Multi Carrier Shipping Label App. The app also takes care of your complete shipping process starting from displaying live rates, printing shipping labels & live order tracking.