Hi Miesha and welcome to the forum,
Get yourself a set of scales and a tape to weigh and measure your items. Depending on who you choose to ship with, most will have auto shipping quote interfaces on their sites where you just enter the details to get the shipping cost. If for what ever reason the shipper you want to use doesn't, then you can just give them the details upfront and they can quote you so you can cover that in your listing.
In regards to knowing if it's a competitive shipping price or not, you can use other services to also quote. You should never just simply accept the first price you find, just shop around and see what the best price is you can get.
In regards to shipping discounts, some companies will offer shipping discounts for high volume regular customers, but you've got to be doing a serious volume before you can even get a sniff at those sorts of deals.
BTW, I like your strategy of picking those types of shops. I would also suggest you don't ignore weekend markets and garage (yard) sales either, they can be gold mines if you have a good eye and can talk a good deal.