The phytosanitary certificate is required by some countries, regardless of quantity ordered or method of posting. Also, if you use a courier, a phytosanitary certificate is mandatory.
This certifies that the seeds were examined and shown to be free of fungal diseases & insects.
To check if your country has any requirements, go to the FAQ.
Most customers with small orders take a chance without this certificate, the price is only worth it for a large order where you stand to lose more if the seeds get stopped. Also, hobby-quantities of seed tend to pass through the system unnoticed.
The cost of this certificate merely covers our expenses in getting it from the South African department of agriculture, and the presence or lack thereof has no bearing on the high quality of the seeds we supply.