The impressions you make on your customers mean everything for your success, and you must make sure that when someone sees your products for the first time, they associate it with quality and reliability, and not a cheap knock-off build that's only designed to get their money. Getting this done is a bit more complicated than it sounds though, and it requires some intricate knowledge of the market for your respective type of product.
If you're designing a small-scale device, you can often improve its appearance quite a lot by ordering some custom-moulded parts that help everything come together. A handheld electronic device is the perfect example for this, as it can look much more professional if you show that the parts have been built specifically for this device.
Sure, nowadays you can use 3D printing as well, but that's not really a viable option if you want to get good results in large numbers. 3D printers are good for small-scale projects, but once you try to mass produce something with them, the scheme can fall apart because they don't work that well on such a level. This makes it important to have a good partner in the market for plastic moulding, as these companies are generally the ones that have the solution you need in such a situation.
Before you even contact the company of your choice, make sure you have your project specifications in order and meticulously organized. There are many things that are going to be important when ordering your parts, from their sizes, to the colors and the number of parts that you want to order. If you're missing even one of those details this can quickly send you back to the drawing board, as the company will want to have the order detailed in every way before they can get started on it. Even a small change can often make a huge difference, so be careful and don't change things in the last minute.
On the other hand, you should also do your own research about the company and know what you can expect from them. Are you looking for a specific type of plastic? Not all moulding companies work with the same materials, so sometimes another company might be more suitable, even if they charge more or lack another small feature that you're interested in.
Also, keep in mind that even though most companies for plastic moulding don't expect a long-term business relationship by default, they would often at least expect you to keep ordering for some time after the first batch, unless you specify otherwise. So if you're just placing a one-time order and have no plans to continue working with that company, make that known in the initial discussions so that there are no surprises later on. Your contract should outline how quickly you can cancel your work with them anyway, so make sure you're familiar with it before signing it, especially if you have more unique requirements like what we described above.