Support In app purchases for multiple platforms

I have been reading about In-app-payments recently and dont quite understand the whole process.

What is the difference between SOOMLA and openIAB.

How can i create one solution for multiple stores i.e. Google play store, iOS, Amazon, Slideme etc.

Both of these allow you to create one solution for multiple stores. However, they have their advantages and disadvantages.
Soomla includes in-app purchasing APIs and inventory management system but openIAB lacks an inventory management system(which is crucial in a game). Soomla’s API design which covers almost every aspect of in-app purchase needs. openIAB supports lot more stores(7 stores including smaller stores such as slideme, yandex) than Soomla, which supports only the major stores. Soomla’s documentation is solid with a dedicated Soomla answers website to answer any query of yours. openIAB is not as responsive as Soomla since the community isn’t that big and any question that has to be asked has to be raised as an issue in their github page.
Edit: Soomla has a knowledge base website which covers any aspect of Soomla.