You might not always receive a receipt shortly after making an in-app purchase. Sometimes you'll get an order for several purchases at once even if they were made on separate days. Grouping up several purchases in one order is one of the ways purchases are billed by Apple. You can find out more information here: https://support.apple.com/en-us/HT201359
Pixonic is not able to overcharge you since purchase processing is done through iTunes by Apple. If you have any questions about the billing process, please contact Apple directly: http://www.apple.com/support/itunes/ww