Encoding numbers that can be decoded mentally? I have a lot of objects, all of which need a particular number tagged on them so I can quickly tell the customer accordingly. But the problem is that the customer should not know what these numbers say (I write how much I buy the object for).

Is there any way I can encode them so that i can quickly decode and understand what the words mean?
I wouldn't try and do this mentally. Whatever you come up with will be insecure, slow and error prone.

Here's what I would do:


Each box would have a QR-code on it that you print and attach to the item. 
You then scan the item with your phone.
Your phone has 128-bit key saved to it that is unique to that device. 
The phone decrypts the QR code and displays the associated data inside the QR code. 


The bigger QR-codes can store enough data that you can actually do the crypto properly with the proper IVs and authentication tags. AES in GCM mode would be my suggestion.

This solution has the advantage that it's very quick to decode the message on the boxes, the underlying scheme is secure, and QR-codes have automatic error correction capabilities, so they can resist damage.

This has to be better than trying to do it in your head.