Printers are not 100% Accurate
If you've ever ordered some custom business cards from a professional printer, they no doubt provided a template that they asked you to use when providing them with your design. The template had some borders that they called the "Safe Edge," and they asked you to make sure that all the important content is kept within those borders.
This is because even the highest quality professional printers cannot be 100% accurate. As with any mechanical process, there are some tolerances that should be taken into account when printing.
When you print a test sheet, you'll notice that the borders do not always line up perfectly with the labels you bought. In fact, it would be very rare that they do. If a professional printer that costs tens of thousands of dollars needs to trim a little off of the edges, you can expect the same for the printer you bought locally, even if it was the best one you could find.
Here are some tips that can help you out in getting your printer to print as accurately as possible.
Printing Test Sheets
Go to the supported labels section of this website and find your label sheet, and download its preview. This will provide you with a PDF that is configured just for that sheet with the borders outlined.
You can save this PDF and print it as often as you need on normal paper while configuring your printer's settings, and getting those borders lined up as close as possible.
Of course, this will only work if you have the same size paper as the labels are on. Even then, keep in mind that the label paper is much thicker than normal paper. Thicker paper will more easily get hung up in the printer, causing it to drift a little as it gets pulled through.
Print Using System Dialog
If you're printing from the web browser, not all of your printers supported features may be available. If you can't get the browser to print accurate enough, use the system print dialog when you can, or just skip using the browser altogether and print using a desktop application, such as Adobe Acrobat Reader.
Use "Label" or "Thick" Paper Type
This is the most important thing you can do. Every printer is different, but most have a specific page/paper type just for printing labels or thick paper. You want to make sure that this page type is selected when you print.
This tells your printer to print with greater accuracy than it normally does, and it may even print slower to make sure that the paper doesn't drift slightly as it pulls the sheet through it.
Make sure that you don't have an option selected to fit or scale to the page. You don't want the application scaling the labels, which will cause some very noticeable issues.
Make sure to look through all of your print options to make sure that none are selected that might end up scaling the labels. Also make sure that auto rotating and centering is disabled.
The bottom line is that you're not going to get the labels to line up perfectly, no matter how much you try. So just get them as close as possible, and remember that the point is to get the parcel to its destination. Your customers are not going to be focusing on this as much as you are. In fact, most of them will likely be too excited to finally get their shipment, that they'll tear it open too fast to ever notice that if their address was 1/32nd of an inch lower it would have been perfectly centered on the label.