You would obviously have your own individual requirements that may be making you to go for a delivery service provider as you may have a bunch of deliveries to make for which it would be safer to look for the right company that you could be working with for a long time. It is not a good idea to keep working with different companies at different times for different deliveries as this way it is not going to be cost-effective for you.
If however you work with a single company for for regular deliveries then you could be qualified for bulk discounts as you would be giving them good volume and that is what they appreciate. The more deliveries you choose to make with a single company the better the rates they would be providing you with. It therefore becomes important that you research companies that offer delivery services so you can only go for reliable service providers that could be trusted for huge volumes of deliveries.
It would not be a good idea to keep using different services every time you discover your previous delivery service provider is not doing the right job for you. Instead of actually giving them a try it would be easier for you to read reviews about them and get an idea from the experiences that previous customers may have had with them