Disgustingly overpriced very dissappointed customer

Horrified that Postnet Shelley Beach charged R20-00 for a single, A4 black and white print from an USB memory stick. There are no prices displayed in-store or anywhere else and we were only informed to pay this amount once the much needed document was printed. Tremendously dissappointed as they have been our first choice in related matters but this will definitely no longer be the case. Is this even legal? Whoever is in charge should genuinely investigate or at the very least give the customer a choice.