which subscription method I should use for flutter

I am developing a mobile POS app using flutter with firebase and I am stuck with the subscription method I should use. 90% of local client will pay in cash or local money transfer (our local market does use that unfortunately) 10% of international payment will pay by basic methods (credit cards) so I suppose google play console could manage that. my question is how to manage subscriptions of local clients (the app will allow them to sign Up) but then after payment is done, I should enable their subscription manually. Thanks