I think it would be great to have a secondary funding source(s), as opposed to the only one funding source, which is manually selected at any time.
Right now, there is only one funding source considered active at any time. Sure, you can switch between the sources manually, but you could automate the process a bit to make the Curve card even more usable. So, the idea basically is that you could select a secondary source (card) that would fund a pending transaction in case a primary (active) one would run out of funds temporarily or fail for any other reason.
Why is this useful? Consider the following scenario. As a user, you probably have multiple funding sources set up (that’s the whole reason of using Curve card, isn’t it). When having multiple accounts at different banks, you must put some effort into managing/transferring your money between them. And sometimes, you just forget to top-up your primary funding source in time and you end up with no balance on that card. Sure, you can (and will) top it up once you realize that the account is empty, but wouldn’t it be great if the Curve curd would be smart enough to draw funds for the immediately pending transaction from the secondary funding source automatically?
Just imagine: you’re in the store trying to pay for your groceries with Curve and your payment gets rejected because your active funding source is empty. You get nervous, you don’t realize immediately what is going on, maybe you entered the wrong PIN, so you try it once again, only to be rejected again. Then it hits you - OMG, I ran out of money on my primary account. Damn. The cashier is looking at you funny, like, what are you trying to pull here? The people in the line behind you are getting impatient because you’re taking too long to pay for your purchase, you feel a bit guilty… Then you pull out your phone, searching through screen pages for Curve app, maybe you mistype the app’s PIN on the first attempt because you’re getting nervous, then you finally get in, and want to change the active funding source. You quickly think through - which other card does have some balance still? Not wanting to get rejected again… Maybe you’re lucky this time and the payment goes through using the other funding source, but all this could have been automatic, without any user intervention…
So, my suggestion is to have a primary and a secondary funding source. The secondary is obviously only used in case the primary source fails (for whatever reason). In this case, a warning message should obviously come up (after successful transaction), stating what happened and reminding the user to check the primary source and top it up if necessary.
Any why stop there? We could have not only one, but multiple secondary sources. Actually, each of the added cards could be marked either as an “automatic source” with its priority level, or “manual source” that never gets automatically accessed, only when manually switched over as a primary source.