*edit, the USPS system has changed. This article has been updated to reflect that.
Type of package:
Contents: Be careful using the word LEGO. I always use "Plastic toy parts". Then under Detailed Description I put "Bulk plastic toy parts". That's sufficient.
Item Value: NEVER add your packaging fees or postage to this. This is only the value of the goods paid for. If you sold $5 worth of parts, your shipping was $2.05, insurance is $1.50, your "packaging" fee is $0.75, and your privilege of ordering from you fee is a brass button, put down $5. Since Europeans and Americans swap a period and a comma for thousands place, round to whole dollar amounts to avoid any confusion or possible $40 customs fees for your buyer.
Quantity: 1. Because it's bulk.
Weight: Whatever your total package weight is including bubble mailer.
Ignore the tariff number and country of origin unless you are a larger commercial seller.
AES Exemption: LEGO pieces are not approved humanitarian aid so this will always be 30.37(a).
Read the Prohibitions and Restrictions and chuckle.
On the next screen verify everything and then print. If filling a customs form for a package (not large envelope), you can pay for your postage all on one easy form. Don't forget to sign and date the form. If you or the postal clerk misses that then your package might get two Transatlantic flights.
Hopefully that was simple enough. If you have any other issues, let me know in the comments and I can update this post as needed.