(Sorry if this is a common question, most of the ones I looked up were from 2011.)
I’m still learning as I go with Unity and have come to a point with dealing with in-app purchases for mobile devices. I am building for Android and IOS and have heard Prime31 (StoreKit ) tossed around quite a bit on the forum while doing some research on this. Are there any alternatives at this point? I do not know Objective-C,
Additionally if I purchased Prime31 for Droid and IOS would I be able to use them both for the same button (“Buy Coins”)? (By having Unity differentiate which device it is running on) Or will I have to make an IOS version and a Droid specific version?
On a side-note can anyone tell me if Prime31 is “worth it?”, I want to do just basic coin functionality. You buy X coins for $0.99 and update the Users SharedPrefs with the new coin amount is basically what I am needing to do. 
Many Thanks,
It may sound simple to just “buy some coins and update PlayerPrefs”. But if you don’t know any objC then yes, a plugin is worth the few bucks :o)
I’m probably gonna purchase the Prime31 for IOS,
However, how would one go about linking to Google and the IOS store? I have done it in Java by simply using the sdk from google. However in Unity do you have to import a sdk or write it all yourself? Would seem a bit odd too have to rewrite it all yourself with how popular Unity is and it not having support.
If it comes to it I will just purchase both but it is always worth the effort to try to save a few bucks 
(in a reasonable amount of time ofc)
Thx again for the information, really appreciate it.
You would likely again get Prime31s SDK for Android too.
He offers them on both side. They will take care of all communication you do with the two stores which you are required to use for any kind of IAP (Prime31 also wrote the Amazon one but they are now offering it on their end and cover the costs).
you can not link the two stores in case you hope to transfer the purchases from one side to the other. you would need an own server backend that stores the related information for persistency across all devices.
Also keep in mind that consumable purchases (like coins) can not be received from the store twice, so once you got them confirmed its up to you to handle them correctly.
I would recommend to not store them locally, otherwise the crackers will be really happy about your game, buying all your stuff without giving you a single dollar at all