I think this might actually be a flow problem. I've rewritten this a bit and added the use of the IsNumeric
and Val
functions which are a bit more appropriate to what you're trying to achieve.
If chkbOther.Value = True Then
If IsNumeric(txtOther.Text) = False Then
MsgBox "Please enter the additional service cost", vbCritical, "Missing Value Error"
Else
Other = Val(txtOther.Text)
End If
Else
Other = 0
End If