Bought a cheap AVR-ISP programmer on Ebay, and you find out that while it does provide 3.3V, it is VCC-only & your device is NOT 5V tolerant on its I/O pins. BUMMER!
This little board fixes just that issue.
All relevant I/O lines (10pin / 6pin ISP) are level-shifted, target power can be selected between 5V (provided by the 5V-only programmer), “unpowered”, and 3.3V (provided by an on-board regulator).
That’s all & it works :-)
Design files and so forth can be found here.