It has to be at least 18 ounces... a few more ounces is fine, and preferred so you don't accidentally go under the minimum; and unless you go WAY heavier, it won't change the cost.
And yes, for the Scenario A shops (where you have to have your own box) I do get the box, packing materials, and contents and weigh them on one of those flat digital platform kitchen scales. I got a really slick purple one at Amazon for $10 or less (can't remember *exactly*). I always go for 19 or 20 ounces, so just in case my scale isn't completely accurate, I'm good on the minimum weight. So far my little purple scale has been very, very close to the Post Office's scale!
For the Scenario B (ReadyPost) shops, where you have to obtain your package while at the Post Office in the process of the shop, and package the items in the post office before mailing... I still weigh my items on the little purple scale. I usually just use a small bubble mailer, so I figure those only weigh an ounce or two. So, I make sure my items weigh the full 18 ounces, and know the bubble mailer will add an extra ounce or two.
If you do like some shoppers do, and always mail the same thing, you'll only have to weigh it the first time. However, I mail doggie food to a gal I know who rescues elderly Shelties and Shelties with chronic health issues that makes them otherwise unadoptable. So usually it's a can of specialty food and some special-ingredient treats to make up the difference in weight.
I was a bit nervous about doing Post Office shops at first, too. But, after doing a couple, they are one of the easiest shops there are. Reports are a piece of cake, too. They *look* long, but it's mostly just checking stuff off with a couple brief narrative boxes.
I always have to make sure I enter before 4:25... once I entered at 4:24, and got an email about making sure to get there in time.
And don't forget, if you do more than one in a day, you must have 30 (or is it 35?) minutes between them.
Oh, and one other thing ~ you need your time of entry (into the building itself) AND the time you stepped into line. So when you step into line, you need to note the time AND start your timing device (mine is the stopwatch on my cell phone). The first time I did one, I forgot to note the time I stepped into line... I just captured the total wait time with the stopwatch. Because every other timed shop (it seems) just wants the total amount of time you waited ~ not the time you began waiting, too. But after that it was indelibly in my mind!
Practitioner of the Nerdly Arts.