The 's can be a little confusing especially when it is used incorrectly even by native speakers as in your second example! It should say CRs - the plural of CR. But it is very common to see 's used incorrectly like this; it shows a lack of understanding of basic grammar. "McDonald's", however, means belonging to McDonald - McDonald's burger restaurant. But it has been shortened to "McDonald's". So your sentence, "I have a package at the receptionist's" is correct - at the place belonging to the receptionist, or the place where the receptionist is.

"McDonald's" is already written in the possessive form. It is what the people who created the company called the restaurant (proper noun) and you have to follow their spelling. Ex. McDonald's french fries are delicious. It's OK to use receptionist's in your sentence with the understanding that you mean receptionist's office/table. But this is a shortcut for native speakers when they speak. If you were to write it in formal English it would be better if you included office/table to be more clear. When it comes to abbreviations, some writers can use apostrophes to show plural nouns. For example CD's and DVD's are commonly written this way and should be followed. However for uncommon abbreviations you should avoid using apostrophes for the plural form.

typo -> what does the 's here mean? is it a manner ...