Please, specify, why exactly you can't pay. 

I can't too in store because paypal do not accept payment from my country and we all know that paypal is not acceptable by all countries but they should be another alternatives to paypal payment because it is limiting some people from buying plugins and themes and it is a big challenge.

thanks

You need to contact plugin/theme developers directly and arrange this with them. I believe they have other options to get money from you:)