Well as far as the weight, I there is some leeway in the way they rate the boxes..for instant, a box weighing from 0-2pounds is one rate and then an additional charge for each ounce over that amount and so on...When you enter the weight it will tell you the cost.. I myself hardly ever just guess at the weight because if for some reason there is an extra charge, they don't charge you they send the box postage due,,and this might not go well with your customer...I suggest just buying a cheap postal scale, it will save you alot of time and money..
also if you are going to ship items that weigh, lets say, several pounds or more, then it would probably be a good idea to just use the flat rate box. You can ship up to 70 pounds for about 8 dollars as long as it fits in that flat rate box..
As far as odd sizes on the boxes, as long as the length and girth dosent exceed 108 inches you are ok at the stated rate..If the box goes over those diminsions, just check on the additional cost on the home page: 'calculate shipping'..Pretty much all the info you need about shipping can be found on the usps homepage and is set up so that you can calculate just about any kind of shipping need right from your home pc...
As far as myself, I normally just drop stuff off at the post office 99% of the time as there are bins you just drop the item into so you don't have to stand in any kind of line. But the few time I have had pick up the picked up the boxes the next day when delivering the regular mail..I just had them setting outside my front door..no problems..I guess it would depend on where you live...