Payment for work done in an emergency situation.

Just water phoned me and asked me to do an urgent installation on a water dispensing machine in Klerksdorp. After giving them discount when asked, they said that they would pay the full amount (did not pay deposit as required) as soon as the installation is complete. I completed the installation the next day, and the shop manager, the owners son, signed my invoice and accepted the work in good order. I immediately sent the signed invoice to the Just water head office for payment as promised. The contact person said that he would immediately arrange payment. This was a friday, 4 March 2016. On Monday 7 March, I phoned and was informesd the financial manager is in Durban, and would pay late afternoon. Thee tuestay she was uot of town, wednesday out of town, thursday and friday in Durban. On Mondaye 14 March 2016 I was told that she is now off sick. It seems that they are looking for excuses not to pay me. I am fed up with false promises, as I was promised every day to be paid the same day or the next. How do I solve this. This has now cost me a lot of money on phone calls and time. Just water is very unproffessional and I would not reccomend any company to trust them.