Customer Stock might sound familiar if you have ever bobbed around a print shop briefly. Customer Stock is material (paper, envelopes, etc) that a customer brings in to have printed on, instead of ordering through the shop. This happens a lot when someone wants to use specialty papers, if someone has a rush order and doesn't want to wait on paper delivery, or if by some odd chance it would be cheaper to bring their own paper.
The number one important thing to consider when bringing in 'customer stock' is always bring extras. It can take anywhere from 1 sheet to 25 sheets just to set up a job... especially if it's being printed on the press. So, if someone is having 500 envelopes printed the smart thing to do would be to bring in at LEAST 525 envelopes... or 550 if they wanted to play it safe!
When paper is delivered to us, or any other print shop, the boxes that the paper come in have all the specific details we need in order to send it through the printer without jamming (size, paper weight, textures, etc). But when a customer brings in a different kind of paper, a majority of the time the package doesn't give the necessary details in order to get the prints going. We pretty much play a guessing game until we get all the right settings. If the settings are wrong, the printer will inevitably jam.
This extra paper is not only for setting up the printer... it's also used for any other special requirements that may have been requested (scoring, booklets, numbering, stapling, drill holes, etc). Depending on what specifically the job entails depends on how much extra paper will be used. For example, setting up the scorer takes more extra paper than setting up for drill holes. Or a job that requires being printed on the offset presses, is scored, and stapled will obviously use more set up paper than a job printed on a digital printer and is only cut down.
Unless you are the person printing, there's really no worry in HOW MUCH extra paper is being used, just as long as there IS extra available. It's all trial and error when setting up, so it's really hard to peg an exact number of extra sheets, that's why (like earlier) we suggest to have at least 25-50 extras.