In the ASOIAF wiki, it states that "almost a year into the siege of Storm's End, Davos took it upon himself to run the Redwyne blockade to smuggle onions and other foodstuffs into the castle".
If he took it upon himself to do this, then clearly no one commissioned him to help the besieged castle. Stannis himself states that during the siege he was all but forgotten about by his brother Robert and the other rebels, so this at least seems to be true. If I remember correctly he also never mentions in any of his chapters that anyone had asked him to help Storm's End.
But why would Davos risk death to help Stannis without any prior incentive?
The only thing that I can think of is gold, but Davos doesn't ever appear to be a particularly greedy man. He also seems to be constantly susprised by the fact that he was raised to knighthood, so again I find it hard to believe that he decided to take such a massive risk for that reason specifically. Every other Lord also appears to be surprised that Stannis decided to do this, so obviously it seems like a much greater reward than would have been expected.
In addition, if no one had asked Davos to smuggle food into the castle, how did he know they were in such a bad way as to require the food he brought? It was probably well known across the kingdom that Storm's End was under siege, but it could have been possible that they had food reserves to last longer than the year in which they held the castle. In which case, how would he even have been sure that he was going to get a reward at all for his actions?
Is there any indication on what Davos' motivation for smuggling food into Storm's End was?