Current and desired level
Pick where the weapon is now and where you want it. Pricing scales with the levels gained, so a partial level-up costs less than a full run to max level.
Attachments
Leveling a weapon unlocks the attachments tied to those levels. A max level weapon has every level-based attachment unlocked.
Add-ons
You can bundle a camo, weapon prestige or a max attachments setup on the same order so everything is finished in one go.