Get a bunch of the free Flat Rate boxes from the post office, the shipping on the larger 3 is $10-$14. If you use Paypal to purchase and print your labels you will save money, the biggest is still only $10 through Paypal. Most parts will fit in those boxes, if the part is only a couple pounds it may be cheaper by the actual weight than using a Flat Rate box, I'll turn the Flat Rate boxes inside out and use them alot of times.
If the parts are less than 13oz.( cdi boxes, shift levers, clutch packs, coils, etc...) they will be first class mail and will usually be less than $5. Bubble envelopes are great for the little stuff.
For the big stuff you'll have to find suitable boxes, I just use a cheap $5 Walmart Fish Scale, you can get a decent digital scale with a 75lb capacity for about $25.
Once you do a couple you'll get the hang of it. Printing the labels online saves a ton of hassle and time.