The phytosanitary cerificate is required by USDA as of 21st September 2001 for seed imports into the US.
This certifies that the seeds were examined and shown to be free of fungal diseases. If you do not include this with the shipment, the USDA may stop your seeds from entering the country.
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.