Regarding shipping, do yourself a favor and buy a postal scale. It doesn't have to be anything expensive. Do a search for "ultraship scale" on eBay. I think we paid something like $25 total (including shipping) for our Ultra 30 scale, and it works perfectly fine for all of our shipping needs.
I'm going to assume you're familiar with how you check shipping rates on USPS? Pretty easy. Just go to www.usps.com and follow the "Calculate Postage" links.
You can either charge a flat rate - meaning everyone pays the same rate, regardless of their geographic location - or you can have a shipping calculator in your ad. Either method is available as an option in the Auctiva listing tool.
"Reasonable shipping" means different things to different people. It's perfectly OK to charge a little extra to cover the cost of shipping supplies (tape, bubble wrap, boxes, etc.), and add that to the cost of your shipping. If you use calculated shipping, you would add this extra cost as a "Handling Charge" when you create your ads. eBay will automatically add the calculated cost of shipping to your specified handling charge, and present that to your buyer as the total shipping cost. If you use flat rate shipping, you need to roll the handling charge and your shipping charge together and list that total as your shipping rate.
There are many opinions on what constitutes as reasonable for including in your handling charge. My personal opinion is that only supplies used directly in the process of packaging an item should be included. I don't consider my time spent on packaging, or gas to get to the post office as part of my handling charge. I take those things into account when deciding on a starting price for my auctions. People have STRONG OPINIONS on this subject, however, and there are those who would surely disagree with me.
I generally think of anything over $2 as too much of a handling charge, and I will issue partial refunds to customers if I feel I made a mistake and charged too much. My items are fairly inexpensive to ship, though. People who regularly ship breakables or antiques will probably charge more to cover the extra cost of packaging materials. Really, the goal is not to make a profit off of the shipping, but just enough to pay for your expenses (again, "expenses" varies depending on who you ask).