Thanks, that’s just what I did.
Here’s their response:
“Thank you for reaching out. The +3 9mm extender does add 2 for the 40cal. The 9mm extender is a little tight on the magazine when sliding it on compared to the 9mm. For the VP40 and the "SK" series guns the extender sits where it is supposed to. The USPc/P2000 there is a bigger gap between the magazine extender and the bottom of the gun compared to the VP9/40 and "SK" series guns. The 40cal magazine is a bit longer than the 9mm magazine in USPc/P2000. The magazine works (shoots/locks the slide back) though with the extender. I hope this helps.“
FWIW, I question the assertion that the 40 cal mag is a bit longer than a 9mm mag. I have both, and either mag will fit and latch in either gun. In fact, I have added distinct markings for the 9mm mags, to avoid inadvertently putting 9mm ammo into a 40 cal gun.