Hey, check a few posts back, as Stackry process has been asked and answered many times.
Briefly, you sign up for Stackry, and they give you a local US address in NH. I will share a link which gives you $10 off your next shipment and I get the same benefit for referal.
Link
As per payment options, they accept Paypal, which I use. I never bothered checking other options.
Once your items are received by Stackry, they inform you by email. They also notify you of its size and volume.
You can then ship your parcel to Pakistan or where ever. If you want, you can also wait or consolidate your items in one parcel to save shipping costs.
Shipping costs are either by weight or volume, whichever is higher. Normally, when they repack the items, it becomes lesser volume, and you end up paying less. Dont worry too much about it now, as Stackry takes care of re-packing themselves.
The packaging and shipping costs are all on their website.
Thats all in brief. Is that enough information to get you started?
Check out my BMW E60 ride thread.