There are various types of delivery companies in Singapore and they usually specialize in different things.
First of all, there are delivery companies like PCA Masters which is a SG local courier. These are companies which perform deliveries locally within Singapore, and are very often privately owned by individuals and entrepreneurs. These usually offer a standard delivery rates for most ad hoc deliveries locally within Singapore, as well as special rates for (usually business) customers which require large volumes of deliveries per month on a consistent basis. Courier services are often engaged by companies which need to deliver urgent and very important pieces of documents.
Next of all, there are international courier companies which have some kind of delivery services in Singapore as well. These are companies such as Fedex and DHL which also have a local presence within Singapore and make deliveries in Singapore too. However, they’re not used as a local courier service provider most of the times. Their services are usually engaged when a customer were to make a purchase from an overseas business and the item(s) were to be delivered to a recipient in Singapore. Their local only delivery services can be quite expensive hence most people only use Fedex and DHL for the overseas delivery services they provide.
There are also postal companies, and the main one would be SingPost. SingPost is the main postal company and they provide deliveries of all letters. This service is obviously the cheapest but is also the slowest. Letters usually only get delivered after 1 or 2 working days, and there is no guarantee that your letter will reach the intended recipient. There is also no form of tracking when it comes to this. In fact, fewer people use postal services these days for sending messages, when Whatsapp is free for most people, and email is also readily available. Additionally, when it comes to very important documents, people trust courier services more, as the items are accounted for and the recipient also has to sign off on a consignment note after receiving the item(s).