How to add text as a normal detail?

You may consider a Displacement Map that distorts the letters in relation to the embossed metal.

1. Open a file

2. Choose Image > Duplicate. In the As: field enter file name as: filename_map.psd. (Example: mailbox_map.psd). This file, which will ultimately serve as the Displacement Map must be in psd format.

3. Choose Image > Mode > Grayscale.  Increase the image contrast using Curves or Levels. Since each image is unique, there is no exact setting. You may choose more contrast than I did. Then choose Filter > Blur > Gaussian Blur. Once again, there is no set amount. The sample above is a useful guide.

4. Save this file – filename_map.psd -- to the desktop.

5. Open another file consisting of a pattern or type. In this case, with type for Layer 1, it must be rasterized. After typing, choose Layer > Rasterize > Type. Drag that image over the full color image. It will be Layer 1. I stretched the type a little.

6. Choose Filter > Distort > Displace. The default numbers of 10 for Vertical and Horizontal Scale usually work very well. For greater displacement,15 is satisfactory, as well. Radio buttons should be set to Stretch to Fit and Repeat Pixel Edges. Click OK.

7. Choose a Blend Mode of Overlay. Adjust the opacity of Layer 1 for the most pleasing effect.

{EDIT} If I were to redo this example I might increase the displacement map contrast and blur a tad more and, at the end, given the type a gentle blur. You may try Multiply instead of Overlay, as well.

Hi

1. Put your text and plain background in a smart object

2. Go to filter 3D generate normal map and adjust the details as required

3. After clicking OK save the document

4. To use that normal map, create a new document and a 3D layer with a material ( I used a simple extrusion below).

5. Then click on normal for the front material and load the normal map you made earlier

The extrusion can of course be from a photograph - so same method, different image and the text was warped before creating the normal map:

Dave

Aaron Nace at PHLEARN  has a good tutorial using Vanishing Filter, Layer Effects and Blur filters. 

How to Add Text to Anything in Photoshop - YouTube

Using blend modes, blend if slider, and styles it will be done. You can find quite a few tutorials on YouTube and on Adobe TV for adding text to an object. Also if the text needs to form around an object like a bottle then warping text may also be needed. Not to mention perspective.

But for a flat door like a mail box, styles will do.