Redirects are normally a 302 to a URL with some query parameters (GET), so it'd need to be something like,

```
?json={encoded_json}&signature={signature}
```

... but http://www.boutell.com/newfaq/misc/urllength.html. I suppose they could make it into a POST, similar to how forms are used to initiate these offsite gateway interactions, but it's more work for them and I doubt they'd go for it. Also, not 100% we'd handle that well out of the box.

Instead, they should send us enough fields in the return URL to be able to create an order, plus a signature.