How and where can I pay this invoice? For instance, freelancers could charge by the word, by the number of images, or by the length of a finished product. How does an invoice differ from a bill? Give each invoice a unique number so it can be easily referenced. For example, «abcdef», «This is a string», and «This is a string with \» inside» are all strings.A character begins with #\ and has the name of the character.