Fisher Price used to make a toy door that had activities and sounds just right for toddlers. Part of the fun for our grandkids was posting letters, but only four generic envelopes are provided.

So, I designed a 3D model of the envelope and created a personal letter for everyone. Every sibling, parent, cousin, aunt, and uncle got their own envelope in their own colour.  The kids loved it, and very quickly associated the names with the different colours.