There is a category of Mukse known as "Mukse Mahamat Hesron Kis"-items that are Mukse because of their value. These items may not be handled for their designated use, because they are prohibited, yet the owner will not use them for any other conceivable permitted use, because of their value. It is prohibited to handle items in this category of Mukse for any purpose- whether for a permitted function, for its place or to protect it. Examples of this category included important documents, such as passports, driver's licenses or a birth certificate. These items are prohibited from use on Shabbat, as they may not be read, and at the same time, they are too important to use for any other permitted purpose. A credit card and bills or receipts that one saves are also Mukse Mahamat Hesron Kis. They are different from a regular sheet of paper, which although is designated a Keli She'm'lachto L'isur, because it is designated for writing, but it may be used for a permitted function or for its place. The Sefer Tiltul Shabbat also includes in this category items which were purchased and one intends to return them. Since they are being designated for return, on will not use it for any purpose, including permitted functions, so as not to lose the opportunity to return it intact. This would be like merchandise which is also Mukse.
SUMMARY
Important documents and items designated for return to a store are Mukse and may not be handled for any purpose.
