I would live with the 10 rounders until the hysteria dies down. 10 vs. 13... Ehhhh... A reload can be done awfully quickly if you practice it.
Now if it were a Glock 17 and we were talking 10 vs. 17, it would definitely influence my decision a bit more.
1o rounds of .40 in that size package is still considerable firepower, BUT...
Have a look at the sticker on the box and see if it originally shipped with 13 round mags. If so, mention it to the shop owner and see what happens... ya never know.