Seasoned wood doesn't mean too much sorry to say unless they have specified that the wood will be at or below a set moisture content. You might also find shopping around that there is no specific quantity for a 'load', 'builders bag' 'ton bag' 'm3' either.. so if you find a supplier that you are happy with at a price you like I'd stick with them. Logs can't be sold by weight else you will always get them at the higher level of moisture when they way more.
For the dehumidifier.... every kg lighter that your logs become is a litre of water evaporated. However a dehumidifier according to the internets uses between 0.5 to 1kwh to remove 1 litre of water... removing 7% moisture could be 30 litres in 1m3, plus whatever household moisture it removes... so say 50kwh per m3.. which isn't insignificant and perhaps explains the advice above to dry outside (numbers from my head, might be off a bit). If you just throw the logs in the coal store as they are then long term (more than this winter) you will be making problems. Last point here is that you will get creepy crawly things coming out to play too....
So for your problem, drying the logs, if you can stack them outside then that is good. Try to find a spot where the wind will blow along them. I tend to stack mine maybe 1 1/2m high (or more) for however long I have logs... and the key point for the winter - 2 or 3 logs deep with the inner logs against the house wall. No cover on the top. So the winter wind will blow through all the gaps between the logs taking the drier winter air with it (winter air has less humidity than summer). The outside layer will get wet, the top 2 or 3 layers will get wet but the others will shed any water that trickles in the stack and will dry more than getting damper. Half way through winter (February sort of) I restock my dry logs with the inner bottom corner of the log pile and that seams to work OK - and restack the damper ones outside, some years some of these also dry enough.... if that makes sense.
20% moisture content....in all serious 22% isn't going to do much different apart from being a bit more fuel hungry as it dries that 2% off in the fire and not the atmosphere (I am never sure about putting logs near the fire to finish drying.... the energy is still being used). If needs must then you can even burn them at 27% but.. you will use more (and it will cost more then). The manufacturers specify 20% or whatever because to sell the stoves they have to demonstrate the stove efficiency and emissions which is easier to do at a lower moisture... so that is specified to keep them right.
For your seller, you could give them a call and tell them that they are not up to scratch as firewood. I suspect the answer will be similar to "we never specified them as 20%" or "we test our logs and everyone we have tested have been at 20%, though it might be the ones you got didn't dry as well as these, sorry!".. but for a refund.... good luck!
All said and done, kiln dried logs are a bit cheaper than gas but that equals out with the extra hassles, split and unseasoned will be cheaper again but an added hassle and straight off the back off the back of an arb truck an extra hassle (cutting, splitting, drying) probably the cheapest but most work. Collecting logs yourself, 5 car loads to the m3? still be paying out maybe £10 in petrol to get 'free' logs depends how far you travel - have you put yourself on the tip site (link above), you might get some logs dropped that way and perhaps that £10 a m3 petrol money could find its way to whoever is dropping them off - £5 notes or a nice bottle of wine and couple of beers appear to be the going rates