Sorry for the long answer, but you did ask a few questions.
Answer 1: I will never choose the Airmail Economy or USPS from my experience. Usually, they use services like Pak Post (or Saudi Post in my case). (See point 2 below). These people are not as professional as the big movers. Even it is more expensive, I will go for DHL, looking at your screen shot of options.
As a general rule of thumb, I found it cheaper to buy and ship several items together than a single item. I have paid far too much shipping for small parcels than a large bulk pack. This is my general advise for all.
Answer 2: Airmail Economy is not a courier company. Airmail is a terminology in shipping for "mail by air". 
They will use multiple usually cheapest partners along the way. Typically USPS in US, then some random airline/s, and then local Post Office. There is no tracking or insurance available in this service for exactly this reason. Trust me, dont put your hard disk in these people's hands. By the time, you receive it, it may be DoA.
Answer 3: I dont have experience of Shipitto, so I checked their FAQ for you. (https://www.shipito.com/faq#methods). They allow Credit Cards, Paypal and Bitcoin (never used it before) etc. They need to have same name on Credit Card with the Account Name. But it does not say anything about shipping and invoice address. Its better to email them yourself, and share the information here for all.
Answer 4: The shipping charges do not include custom duties. These will ne decided by Pak Customs and the parcel service will pass these charges to you, when you receive the parcel.
Answer 5: I am not sure Stackry needs a verified Paypal account. They do allow a payment via Paypal or bank transfer. And if you have credit card, you can open paypal account. Add your home address to it as main address, and thats it. Now you have a working Paypal account!
HTH.
