Hypothetical; no prices or delivery times are given, since GTBuy quotes them per parcel.
Alex's parcel
The exchanged hoodie, a pair of trousers and a power bank are all approved in the warehouse.
Problem: the power bank
Alex notices that some lines do not accept batteries. Rather than lose the choice of lines, Alex leaves the power bank for a separate parcel on a line that allows it, if one exists for Alex's country.
Packaging
The trousers came in a large box. Because carriers often charge by size as well as weight, Alex removes the box.
Address
Alex types street, flat number, postcode, town and phone number as the local post would.
Choosing a line
From the lines GTBuy shows for the destination, Alex compares tracking and the quoted price, then pays.
Waiting
Tracking goes quiet for a few days during handovers. Alex keeps the parcel number in case GTBuy support needs it. Before paying, Alex had also checked the coupon page for a shipping coupon.
Arrival
As a parcel entering the EU from outside, it is usually subject to import VAT, sometimes duties, and possibly a carrier fee. Alex checks the national customs authority's website.