Shipping size and more options? what dose it mean

Level 3

       Hello. I have lots of shipping questions that don't seem to be answered. So is there any way of selecting a size for shipping. It seems like we can only add the weight but nothing with the size. How can this even work. I could be shipping something that is only 1 pound but 40 by 40 by 1 inch or something that is not many pounds but is huge. 

       The reason I ask is I am selling my art. The problem with art is like drawings/paintings and what not is they are low in pounds but large in size. I offer shipping on some but feel I could be hurting myself if the shipping price is based on pounds and not specific to size. It has to be specific to size in some way.  Also can we combine shipping. What if say a buyer decides to buy 2-5 things from the same person at the same time. Will the shipping technology account for this. Will it charge them more or understand we the seller would be putting everything in one box. What if we want to insure the shipping or offer something extra. Will we have to pay for that or would that be up to the buyer. I feel we should insure our shipments. Also is there a way to give the customer money back. This would be in case  we overestimate.   


      Oh another question. How long is our grace period in terms of getting are shipment to the post office. Is it assumed that we must get it in the day the buyer buys it or is it understood it may take a day or so. Also what if its Sunday or late Saturday and the post office is closed. Also can we ship outside the post office or must the shipments be in the post office and not through fedex or something(I feel some shipments may be cheaper with fedex or UPS)