That's so shit that they take so long to pay and there really is no good reason for it. Maybe put in the terms that the payment must be within 20 days or there will be some kind of extra charge?

I work in music and it's even worse, everything takes at least a couple of months and certain royalties take years to come through and there's really no good reason for it. Maybe the blockchain can be used as a potential solution for the problem some day