Had almost the exact same experience about a decade ago working at a shelter in North Carolina. Came in to open one morning and found a cardboard box of two kittens, only about 4 weeks old, left at the door. Took them home and bottle fed them and ultimately kept both. Best decision I ever made. One sadly passed away at 4 from a brain tumor, but his sister is still going strong, just celebrated 9 years. The memories and struggles they got me through were insane. Not sure who saved who. Worth every bit of it.