Whats wrong with my error trapping function? - vb.net

Question

I have a program for buying shirts, I'm changing it so the error trapping happens through a function, so I deleted my previous error trapping, created the function, and tried it out. But somewhere I must of screwed up, because it's not working properly. I was following step by step the video guide from my class, but I can't find where I went wrong. I've been trying to figure it out for a few hours now but have gotten nowhere. Any help would be appreciated. What's happening is when I click Calculate or Next Sale, it will always give me my error message "Please enter a valid number" I've tried entering 0, 1, 25, 50, 51, 55, and 5a. It should give me an error message for 5a saying "Please enter a valid number", because it's not numeric. And it should give me the error message "Please enter a number from 1 to 50" for 0, 51, and 55. I couldn't even get that error message to work because every number I entered gave the "Please enter a valid number" error message. Where did I go wrong? Help me find the error please.
Here is the function:
Private Function CheckTextBox(ByVal TextBoxName As TextBox, ByVal Min As Single, ByVal Max As Single, ByVal IntCheck As Boolean) As Boolean
If Not IsNumeric(TextBoxName) Then
MessageBox.Show("Please enter a valid number", "Error", MessageBoxButtons.OK)
TextBoxName.Text = ""
TextBoxName.Focus()
Exit Function
End If
If Val(TextBoxName.Text) < Min Or Val(TextBoxName.Text) > Max Then
MessageBox.Show("Please enter a number from " & Min & " to " & Max, "Error", MessageBoxButtons.OK)
TextBoxName.Text = ""
TextBoxName.Focus()
Exit Function
End If
If IntCheck Then
If Val(TextBoxName.Text) <> Int(TextBoxName.Text) Then
MessageBox.Show("Please enter a whole number", "Error", MessageBoxButtons.OK)
TextBoxName.Text = ""
TextBoxName.Focus()
Exit Function
End If
End If
Return True
End Function
Here is the code for the Calculate button:
Private Sub btnCalculate_Click(sender As Object, e As EventArgs) Handles btnCalculate.Click
'Local Variable Declaration Section
Dim sngSalesTax As Single
Dim sngOrderTotal As Single
Dim intQuantity As Integer
Dim sngItemTotal As Single
'Data Input Section
If msngShippingCost = -1 Then
MessageBox.Show("Please choose a shipping method", "Error", MessageBoxButtons.OK) 'Displays an error messsage when no shipping method is chosen
Exit Sub 'Terminates the click event to allow shipping method to be chosen first
End If
If CheckTextBox(txtQuantity, 1, 50, True) = False Then
Exit Sub
End If
intQuantity = CInt(txtQuantity.Text) 'not sure if this line is needed?
'Calculation Section
sngItemTotal = msngItemPrice * intQuantity
msngSubtotal += sngItemTotal 'Calculates the subtotal
sngSalesTax = msngSubtotal * csngSalesTaxRate 'Calculates Sales Tax based on the sales tax rate constant
sngOrderTotal = msngSubtotal + sngSalesTax + msngShippingCost 'Calculates total for the sale
mintOrdersPlacedToday = mintOrdersPlacedToday + 1 'Calculates the number of orders placed today, adds one to the previous number
msngTotalOfOrdersToday = msngTotalOfOrdersToday + sngOrderTotal 'Calculates the Total of all the orders placed today
'Output section
lblShowSubTotal.Text = FormatCurrency(msngSubtotal) 'Displays the Sub Total
lblShowSalesTax.Text = FormatCurrency(sngSalesTax) 'Displays the Sales Tax
lblShowOrderTotal.Text = FormatCurrency(sngOrderTotal) 'Displays the Order Total
lblShowOrdersPlacedToday.Text = mintOrdersPlacedToday 'Displays the Orders placed today
lblShowTotalOfOrders.Text = FormatCurrency(msngTotalOfOrdersToday) 'Displays the Total of the Orders placed today
lblShowShippingCost.Text = FormatCurrency(msngShippingCost) 'Displays the total of the shipping cost
End Sub
And here is the code for the next item button:
Private Sub btnNextItem_Click(Sender As Object, e As EventArgs) Handles btnNextItem.Click
'Local Variable Declaration Section
Dim sngSalesTax As Single
Dim intQuantity As Integer
Dim sngItemTotal As Single
'Data Input Section
If CheckTextBox(txtQuantity, 1, 50, True) = False Then 'Uses the function to error trap exits sub if functions returns as false
Exit Sub
End If
intQuantity = CInt(txtQuantity.Text)
'Calculation Section
sngItemTotal = msngItemPrice * intQuantity 'Calculates item total
msngSubtotal += sngItemTotal 'Calculates the subtotal
sngSalesTax = msngSubtotal * csngSalesTaxRate 'Calculates Sales Tax based on the sales tax rate constant
'Output section
lblShowShippingCost.Text = ""
lblShowOrderTotal.Text = ""
lblShowSubTotal.Text = FormatCurrency(msngSubtotal) 'Displays the Sub Total
lblShowSalesTax.Text = FormatCurrency(sngSalesTax) 'Displays the Sales Tax
End Sub