Dear avsking,
Thank you for sharing your concern. You can add different products from same seller and can proceed with a single checkout. As of now, this facility is not available to combine products from different seller as once you pay using PaisaPay - a unique PaisaPay ID generates - that denote you and the seller.
Hope, this answered your query. feel free to revert, in case you need more clarification! I would be happy to assist you 🙂