You have your closing parenthesis in the wrong place causing the first
to be always true.
I also added a line of code to remove spaces from front and back of
in case user accidentally types spaces before or after the zip code.
method is for internal use only, You do not need it and should not use it in your code. See the documentation for Label.Show
ziptextbox.Text = ziptextbox.Text.Trim
If shippingonelistbox.FindString(ziptextbox.Text) >= 0 Then
shippinglabel.Text = "Shipping is $15"
ElseIf shippingtwolistbox.FindString(ziptextbox.Text) >= 0 Then
shippinglabel.Text = "Shipping is $20"
shippinglabel.Text = "The zipcode was not found!"
Tested: Visual Studio 2012