Physical copies are often cheaper than digital. Check:
You cannot simply click an NSP file on a standard, unmodified Nintendo Switch and play it. The console has a strict cryptographic signature verification process. To play a "free" NSP file, a user must:
While this doesn’t include Ben 10 games, the subscription gives you access to hundreds of classic NES, SNES, N64, and Sega Genesis games – a legal alternative to ROM sites.
While not a "system seller," the game is often regarded as a solid entry-level title for children. It lacks the depth of AAA action games but serves as a faithful interactive episode of the cartoon.
GameFly (in the US) rents Switch games by mail. You could play and beat Ben 10: Power Trip! in a month for ~$15.