In Systematic Sampling each element has an equal chance of being selected, but each sample does not have the same chance of being selected.
Here, the first element of the population is randomly selected to begin the sampling. But thereafter the elements are selected according to a systematic plan.
If the previous retailer uses systematic sampling she would proceed as follows:
She would randomly select one bill (or customer), say bill number 6. Thereafter she would select subsequent bills (or customers) according to a systematic plan. Say, she would select every fifth bill so that the selected bills (or customers) are bill numbers 6, 11, 16, 21 and so on.
Notice that the sample containing bill numbers 6, 7, 11, 16, ... will never be selected in that form.
The main advantage in using Systematic Sampling is that it requires less time and lower costs than Simple Random Sampling method. But by using Systematic Sampling method the problem of introducing an error into the sample is more.