Free ways to merge MP3 files

Free Merge MP3. Add the songs, arrange them, and press Merge. Smart mode copies compatible MP3s when you leave trims and fades off.

FFmpeg. A concat list also copies matching MP3s, if you are comfortable writing the command. You do not get the waveform trim window.

Avoid merging by importing into a video editor and exporting MP3 again. That encodes twice.