Here is my solution. You can make coding easier if you set the Tag property values during design and not at userform initialize as I did. I would pick the 12" size as True for the default so that you don't have to check for one option as True as I did.

Private Sub UserForm_Initialize()  
  Dim c As Control
  On Error Resume Next
  For Each c In Controls
    c.Value = False
    Select Case TypeName(c)
      Case "OptionButton"
        c.Tag = 8
      Case "CheckBox"
        c.Tag = 1.5
      Case Else
    End Select
  Next c
  optFifteen.Tag = 9
  optEighteen.Tag = 10
End Sub


Private Sub cmdOrder_Click()
  Dim c As Control, d As Double
  If optTwelve = False And optFifteen = False And optEighteen = False Then
    MsgBox "You must pick a size."
    Exit Sub
  End If
  
  On Error Resume Next
  For Each c In Controls
    If c.Value = True Then d = d + c.Tag
  Next c
  
  MsgBox "Total Cost is: " & Format(d, "Currency")
  ClearControls
   With Range("B7")
     .Value = d
     .NumberFormat = "Currency"
   End With
End Sub


Public Sub ClearControls()
    ' clear all controls
    Dim c As Control
    On Error Resume Next
    For Each c In Controls
      c.Value = False
    Next c
End Sub