How do I read the 上? うえ? or じょう?
What is the meaning of 上 in that sentence?
「上」 is read 「うえ」 in that context.
「～～の上（で）」 means "based on ~~". The ~~ part acts as a premise or condition in performing an action. The ~~ part usually describes an understanding or agreement of some sort.
= "Please place orders upon agreement and acceptance of (what has already been explained)."