Zooz may be a good solution for you. They enable merchants to accept payments globally. They are highly scalable and modular and help merchants to manage their retail channels, payments infrastructure, and third-party integrations on one platform via a single API connection.
You can also look at Paymentwall which provides multiple payment options like mobile payments, credit cards processing and popular e-wallets, all under a single API. It is essential to have more than one payment method to conduct a large number of small transactions.
PayPro Global is another option that offers special packages for start-ups. They have dedicated 24x7 customer support and offer product management, sales and marketing tools and plenty more features for business development.