I'm not sure what the opposite would be- non payment, probably.
1. We are very sorry that a mistake was made in dealing with your order. We arranged to dispatch a repeat order to you today.
2. Because the stock of this item is limited, we suggest you place an order immediately.
OK, but if you want to use a less direct phrase 'at your earliest convenience' is a polite way of telling someone to hurry up. I would also use 'stocks ...are'
3. I am currently working for LSIS Co. as an Senior Manager in the Internal Certification TFT, and I am mainly responsible for internal-certificating work.
a Senior Manager. What do you mean by 'mainly'- are others also responsible or do you have other responsibilities?
4. I am happy with your gift for giving me cyber money. with this money, I bout clothes for my avatar.
I was very pleased to receive the cyber-money you gave me and bought clothes for my avatar with it.
Student or Learner