Mail is called “lumpy” anytime it contains an object that makes it stand out from a flat letter, flyer, or postcard. Any box you receive could be considered lumpy, though the term is usually used when discussing direct marketing products.
Moreover, what is 3D mail?
3D Direct Mail, No Silly Glasses Required
No matter what you call it—dimensional mail, tactile mail, or (our preference) 3D direct mail—packages that feature height, breadth, and depth consistently outperform old-fashioned “flat” mailers. At Tactive, we can customize a winning 3D direct mail campaign for your needs.
Considering this, can you send bulky items in an envelope?
Speaking of odd shapes, don’t mail bulky, odd-shaped things like pens or bottle caps in regular letter-size envelopes. Not only will you pay more in postage, but these items are likely to poke through the envelope, fall out, and be lost — they could even damage postal equipment or hurt someone.