Double Check

Its a simple app, but it does a very crucial part of our fulfilment process and never failed. Since we used it, it made a huge difference in our fulfilment process. Arguably it is not cheap (given the app is really quite basic) but the savings it does in time and errors greatly outweighs the cost.
There are a couple of other apps out there that do a similar process, but Doublecheck was the right one for us and most intuitive. In any event, it is always good practice to trial a few different apps to see what works best.
Did not have any issues with support, but realistically we only had to contact a couple of times for a few question but the app always worked.

Great app and fantastic customer service.

This app has become an integral part of our fulfilment workflow. We have been using Double Check for a few months now with amazing results. It has cut our shipping errors to zero for the past three months and sped up our whole pick and pack process.

We haven't had to contact Avia support too much but the one time we did support was fast and accurate.