The convenience issue is definitely the sticking point with any reusable container concept. I think a cost incentive will overcome this barrier for most people, though - and I'm fairly certain that single-use sachets are much more expensive per gram of product than buying in bulk. The question, then, is how to support low-income consumers to adopt the bulk-buy in reusable containers system? It seems one of the advantages of single-use packets, besides convenience, is low-cost because some consumers do not have the level of cash-flow required to buy in bulk - they only every have enough cash to buy their daily needs. Perhaps the reusable container concept could be combined with micro-lending or a subscription based model to help those low-cashflow consumers to adopt the system?
I absolutely agree that reusable containers need to be part of the solution. Some consumers already employ these habits - they take their own jars etc to local markets and co-op stores to buy mostly dry goods (flour, lentils, beans etc), cleaning products (washing powder, dish-soap, shampoo, conditioner etc) and vegetables. This behaviour is mostly limited to these types of retail outlets and particularly environmentally-conscious consumers. Why haven't these practices gone mainstream yet? Perhaps there is some scope for research on both the supplier and consumer side as to the benefits and problems of such an approach. If we can understand why this behaviour hasn't already been taken up in the general community, we might be able to design a system so that it is more easily implementable for general consumption. For example, what are the drawbacks? Is it inconvenient? Does it require more time due to the need to wash containers? What would be the benefits of using multiple-use containers for consumers who aren't particularly environmentally conscious?