Third party payment online ?

Question

Hello, we have some android 7 device with an app (xamarin android) we have made .. We d like to add a payment option NOT FOR TO USE this app, but for delivery some service image this sample : user arrive at the client and client need to pay ... I think the best way for me, it's just to only have a third party app for payment by card ? like this, we don't manage money in cash ? Could you have some return of experience perhpas about some SDK or anyting else ? we begin to analyse and we would like to have some information ... thanks for all