Correct usage of lbs. as in "pounds" of weight

What is the correct way to say: "All items over 5 lbs. are excluded." I'm specifically asking about "lbs." or is it "lb."? American English if it matters.

Also is "5lbs." ever correct? Or is it "5 lbs."?

In scientific publications, units of measurement are almost never pluralized when abbreviated. This should remain true for general use, as well. You should therefore never write "lbs." You should technically not need a period after "lb" either, unless it's at the end of a sentence.

The abbreviation "lb" comes from the Latin libra, which is itself short for libra pondo, or "pound weight." And in any case, the plural of libra would be librae, not libras.

And, again in scientific papers, there is always a space between the quantity and the unit.

Grammar Girl has a related article, “Units of Measure.” The key points:

• Put a space between numbers and units of measure. Typographers prefer a thin space, but you shouldn't generally need to worry about it.
• Abbreviations are generally the same for singular and plural units: While lbs isn't incorrect, lb is better for formal and scientific use. Never pluralize metric system units like meters and grams.
• Period usage varies with style guide. They're more common in American writing. However, never use periods with metric system units.