hi rasputin, i operated a 'publishing' business - completely unrelated to toys of any sort - for 12 years. in addition to working with shops and distributors, we did a great deal of mailorder so i guess i have some experience with the logistics of shipping and streamlining the process as much as possible.
domestically, the usps continues to fare poorly. rates will continue to rise and specialty services will go up. it remains a good way to move packages and, often, better than their reputation suggests. it is always worth considering the alternatives, however, especially on heavier items. fed ex is a better value than ups but not always as convenient depending on your location.
if using usps, always ship with delivery confirmation when shipping domestically. it puts a little more responsibility on usps although if a package goes missing, it's missing and, at times, a carrier doesn't scan the number.
if you anticipate shipping a lot on a regular basis, there are a handful of companies you may want to look at working with to buy your postage such as endicia.com. where i live, the post office is notoriously inefficient with many unfirable employees who only contribute to the general frustrations of going into their physical shop. we found early on that doing everything from our location was the best way to go. automating your shipping and customs expenses is key. companies like endicia help to facilitate this. i am sure there are many others now as i has been a few years since we shut down.
invest in a thermal printer. you can use this to print your entire address, customs form, postage and delivery confirmation. many refurbished thermal printers are available second hand.
a scale, of course. they are cheap and widely available. they all come from china. you don't need a branded digital scale. thoe are all just rebadged anyways. i bake quite a bit and my scales are essential to making the bread making process go quickly. i will, at times, use that same scale for items up to, i believe, 20lbs.
there are usb scales that you can plug right into your computer. this could allow you to skip the process of committing a weight to memory or writing it down before preparing your shipment.
the postman is your friend. if he isn't, make it so. people are people and if a bond is there to be forged, your carrier may be willing to take your shipments to the post office during his mail route. this will save you a trip.
if you do have to make that trip, bring your goods around to the back of the post office. you may be met with stares at first but if you indicate that these are aleady metered packages, someone should advise you where to place your goods.
as for packaging, reusing is good for the environment and the wallet. double walled boxes are best - this, for instance, is what most electronics are shipped in. alternatively, you can double box but this will add to the overall weight and expense - especially for overseas shipping.
if you expect to be shipping a lot of single box 'special' figures, it may not hurt to invest in some new boxes. hardbound book mailers would be perfect. for a start, try looking at uline.com. this will give you an idea of pricing and they have a presence all over. similarly, a roll of bubble wrap is also a wise investment.
at the end of the day, you may end up tacking on $1 or so for these packing materials but i'm sure people would welcome that over a damaged box.
finally, as for co-ops, i am a member of one and the participants will also tack on paypal fees if this is principally how you expect to get paid. ultimately, with fees and charges in this environment, i suspect the goal is true transparency so nobody feels gypped.
i hope this helps.
nunney