Because of liability, they probably won't ship to you in NY. Anyone can say they are going to make the collapsible stock PINNED, and then NOT do it.
You can call and explain and ask they send you one, but likely they won't. You may have to find an out of state address to have it shipped and go and pick it up.
As well, it wouldn't hurt to find a NY gun forum and ask the question about doing a pinned stock yourself, or even better, look up the law and read it. You never know if they make it illegal to even posses it for a short time before the pinning is performed. An FFL/Gunsmith could probably answer the question for you, as well, they may be able to order the collapisble stock for you and get it because they are an FFL/Gunsmith, and the sender will be confident that it won't be used illegally. Of course the FFL/Gunsmith will probably insist they pin the stock themselves, to ensure you don't use it illegally, so there will be additional fees.
I have to have my 30rd Mags sent to my brother in NC, in Maryland >20rd magazines are illegal for sale/xfer/manufacture, but legal to posses. So you have to buy them out of state and bring them back with you, no one will ship >20rd mags to MD, no matter where the sale was made, shipping a MAG to MD direct from seller to buyer, can be interpreted or misconstrued as selling it in MD.