Back in 2000 when I first started Selling on Ebay, they didn't have the Shipping Cost Calculators available today, and Believe me, they make life Much Easier to be sure. However, you still need a reasonably accurate scale in your shipping area, and remember it's always best to pre-weigh the proposed Sale Item along with the box and packing materials. You don't have to seal it up, just know what the Total weight is going to be. It's actually best not to pre-pack and seal, in case there are questions or additional photos requested.
Now, about the Scale. Staples, Pitney Bows, etc, they're all out there, for around $100+. I went to a Used Restaurant Supply Store, purchased a Wonderful Digital Scale for weighing Produce/Meats etc. for $30. Now, it's no longer State Certified/Inspected, but wow, it'll weigh in up to 99 pounds, and certainly does the job for calculating shipments. A good "weigh" to save money and get the job done. Best Of Luck On Ebay!