Adding Problem, not sure why?

I'm bringing values from across forms into one summary form for my project. They are all coming in, yet when I try to add the three number values into one total price box, it only takes one of the values. Any idea why this might be happening? I have everything declared on a module as globals, and the code to bring in the values is on the respective forms from which they come from. Here is my code on the summary form to add the three:
Public Class frmOrderSummary

Dim decTotalRentPrice As Decimal

Private Sub btnFinish_Click(sender As System.Object, e As System.EventArgs) Handles btnFinish.Click
'End the program
End
End Sub

Private Sub frmOrderSummary_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

decTotalRentPrice += decInsurance + decVAT + decTotal
lblDisplayTotalPriceOfRent.Text = decTotalRentPrice.ToString("C")

End Sub
End Class


Thanks