The shipping label on your package will display the shipping method and weight, as well as the seller's zip code. With this information (and you may have to measure the package dimensions) you have everything you need to calculate the actual postage cost. Both the eBay website and the USPS website have shipping calculators which allow you to determine the true cost of postage.
Now, it is common for sellers to charge an extra dollar or so for supplies - boxes, bubble wrap, etc.
But do not accept a $5 overcharge because the seller "had to buy the tape". It is also against eBay policy for a seller to charge for gas, mileage, or time at the post office - do not let the seller off the hook if this is the explanation you get.
If you receive the item and determine that you have been overcharged for shipping, unfortunately there is no way for you to receive any sort of refund or reimbursement (unless the seller willingly agrees to this). But there are other actions you can take to let eBay and it's buyers know that the seller overcharges.
1) Contact the Buyer - if you feel you were overcharged for shipping, the first thing to do is share your thoughts with the seller. Many sellers want to work with you so that you are satisfied with the transaction, but they won't know there's a problem unless you make them aware of it. As always, being polite will likely get you much further than being rude in all communication.
2) Leave Negative Feedback - feedback is not to be abused, but if a seller overcharges you by a large amount, I recommend leaving a negative feedback comment (be sure to state that the reason for the negative is related to the shipping cost). For less serious overcharges, a neutral feedback may be more appropriate.
3) Leave Low DSR - the Detailed Seller Ratings are very important, especially to Powersellers and Top Rated sellers, many of whom receive discounts on eBay fees if they stay above a certain rating. They often claim to strive for 5 stars, but you should leave less than 5 stars in the category of "Shipping and Handling Charges" if you were overcharged.
4) Report the seller to eBay - it is important to let eBay know if a seller is abusing the shipping and handling costs. Many sellers overcharge in an attempt to recoup some of the fees that they must pay eBay and Paypal when they make a sale. Do not let this happen to you! Click the following link and Report the Seller to eBay - scroll to the bottom of the page where it says "Contact Customer Service" - report the seller for "excessive shipping" or "circumvention of eBay fees".
I am both a buyer and a seller here on eBay. I don't want to see sellers take advantage of buyers. If you feel you have been charged excessive shipping, please try to work it out with the seller first. But if the problem is not solved, used the steps above to take action.