It's maybe a bit irritating to you Tom, and the stuff should ideally have been delivered to you in a covered vehicle. I'd say the company owes you a small apology, but I wouldn't expect to get your money back
I wouldn't get too excited about the rain that fell on it. Wood, being hygroscopic, meaning it gains and loses moisture in an effort to reach equilibrium moisture content (EMC) with the relative humidity (RH) of the surrounding air. Wood has another important property, i.e., the hysteresis effect. This, simplistically, means there is a time delay between a change in the surrounding RH (or actual water on the surface of the wood) and a change in the wood moisture content (MC). There's a time lag.
Even heavy rain for half an hour will have very little real effect on changing the MC of the wood, and any significant change would be restricted to no more than 2- 4 mm deep. To fix the problem (if you haven't already done so) is immediately sticker the planks up so that dry air can circulate around them to remove the shallow penetrating surface moisture. Two or three days stacked like this should dry out that outer shell to what it was before it got wet.
A second reason for immediate stickering is to get the surface MC down below 20% as fast as possible to prevent the slight chance of the dry rot mould (specifically) getting a grip. All other moulds need much more water than this for a great deal longer for the spores to hatch, but 20%MC in the trade is known as the 'dry rot safe MC.
After you've dried off the surface moisture I think you'll find you can just get on and use the stuff. Slainte.