Debug Issue with place order in ecommerce portal

In a working on a Codeigniter based Ecommerce portal, on selecting multiple items for the cart, we are facing 2 key issues: 1. Facing error if we have multiple sellers selected. 2. Facing quantity increase in cart when selecting a variant. You are required to take out machine on teamviewer/anydesk, find and debug the issue Bid only if u can start right now, work within the specified budget, can work on teamviewer/anydesk.