THE worst pistol magazine for the P99 [assuming that USA brand isn't making them]. Run away from them as fast as possible.
Stick with the S&W magazines for an inexpensive price.
I was stuck with two ProMag magazines when I bought a SiG P229. Every magazine full failed to feed at least twice per use. I use them now ONLY for training new auto pistols shooters in clearing malfunctions. They are ok for that purpose.
I have never had good service out of ProMags.. Every few years I go brain dead and try them again, but never with 100% success.. In their defense they are very good about replacing defective magazines, but replacing a defective magazine with another defective magazine isn't that great a deal.
CDNN sells factory original (MecGar mfg) 15rd Walther Mags quite reasonably, but even better sells the 16rd mags marked SW99 for even less.. I plan to buy two new SW99 mags from them and download to 15 just because.
You might want to check them out.
You are the owner of a high quality, relatively expensive pistol.. Why save a few bucks on an inferior aftermarket magazine? I wouldn't use an aftermarket mag in any self defense firearm I use.
Walthers are historically extremely reliable. That's one reason I have four of them. I don't want to waste that reliability to save a few bucks.
I have been involved in shooting sports approx 40 years, over 10 years as a firearms instructor. Have purchased several Promags and found them to be of poor design, substandard material and poorly fabricated. I would now not even insert them into a quality pistol like a P99.
On the other hand, the original Walther mags made by MecGars are some of the best mags in the world, with those available, no reason to even consider stuffing a substandard product into the mag well of such a fine firearm. I stay away from Promags and I also stay away from any firearms dealer that sells them, as I believe it does not speak well for a dealer.