If you’re selling online, yes, you need a digital scale.
A lot of beginner sellers start out without having a proper postal scale. They are either overpaying for postage by overestimating the weight, or having to go to the post office to have their items weighed and paying full retail postage prices, having their items returned or sent with postage due, or they'll stick to only mailing in flat rate boxes and envelopes (which means they are overpaying in many cases). You will see posts in online reseller groups asking “Do I need a scale.” Yes. Yes, you do.
First off, a good postage scale does not have to be expensive. And, you might already have one – a good kitchen scale will be accurate enough to weigh packages, but typically don't have very high weight limits. Even with a decent kitchen scale, you'll eventually want to upgrade to a scale that can weigh heavier items – say up to fifty or even 100 pounds.
A good postage scale must be accurate to a tenth of an ounce at all ranges. Some scales (like this one) market themselves as "postal scales”" but are only accurate to a full ounce once the weight is over one or ten pounds. This is not ideal due to the way postage is priced, which is up to the nearest pound. For example, a Priority Mail package that weighs ten ounces gets charged at the same rate as a 1.0 pound package. But, if the package weighs 1 pound, 0.1 ounces, it will get charged at the up to 2 pound rate. Yes, many times you can get away with it, but the post office can and does return mail for being tenths of ounces over the postage paid.
So, the scale you want to buy needs to be able to weigh heavier packages, and to be able to weigh things to the nearest tenth of an ounce. There are also a few nice-to-have options for your scale. One is a scale that can run off batteries or off a power adapter. This gives you flexibility to easily move the scale around and also gives you an option where you aren't using up a lot of batteries. Another really great feature is a scale that has a display that isn't directly on the scale itself because often you will weigh boxes that are big enough to block the display.
Now, maybe you are thinking this is going to cost an arm and a leg and you're just getting started and don't have a lot of extra money to spend. The good news is you can get a decent scale for $20 or less on Amazon or eBay (although you need to be careful what you buy at the price range, there are some models you don't want). It will pay for itself very quickly in postage savings or gas savings from driving to the post office. The scale I actually use and recommend (link) costs around $26 and it has all of those nice-to-have features I mentioned- dual power and remote display read-out- and can weigh up to 110 lbs.
So, go do yourself a favor and get a scale.