If I understand the system correctly, there will be 2-3 orders per day which you can complete with stuff you have, another few where you have most of the items but need a couple of things from the available contracts, and at least half the orders are effectively impossible--just there to make it look like there are more than 5-6 orders per day.

I usually do the already-complete orders [for non-poor, non-common clients] and ignore the rest. If my warehouse starts to get full, I ignore contracts for a few days and just fill all the orders I can and reset the days (pretty easy to more than cover Warehouse fees, even on a poor orders day) until I feel like I have space again. Pretty lucrative, especially if you only bring back Premium (or Rare & Premium) items to your warehouse.

Also good to know: As of 1.0.7, you can still over-deliver on ordered items and get paid at the same per-item rate. So if someone orders 1 Premium Table and you have 5 Premium Tables in stock, send all 5 and get paid 5x as much. I only do this for the more lucrative orders (never poor or common clients), but loading up a truck with dozens or hundreds of walls & planks for a wealthy client who wanted a few of each? Amazing.