Does Roku allow us to fulfil the same type of requirement like payment for a group of the items that are added in cart or buy an individual item using by "instance purchase" respective item button. Please suggest.
Do you mean from technical standpoint? If you are using RokuPay, it doesn't (yet?) support dynamic pricing. If you're using your own backend billing system, then I don't see why not. You might be able to even use a hybrid where RokuPay could be an option for individual items but groups would need to pay through your system.
But as far as what Roku allows / doesn't in publicly certified channels, you may need to contact someone there.