Not poor service but no service

I have got an online buy and sell company . For years i could depent on sa postoffice for delivering on time or maybe a day or 2 late , but fir the last six months there is no service at all . Stuff witch i have orderd 3 months back is still heldup in the hub . I have sent emails to complaine but the only reply i got this far is that it will be deliverd . This was almost a month back . Now my customers (with whom i have been dealing for years ) want their money back because they dont get the stuff they have orderd . This is going to force me to shut down my company and put 4 people out off work as result off this