Hi and welcome to the forum.
This should get you started:
Private Sub BtnOK_Click()
Dim rw&, lr&, dys&, amt&, msg1$, msg2$
If ListBoxCustlist.ListIndex = -1 Then MsgBox "Please select a customer.": Exit Sub
If Not CheckBox1 And Not CheckBox2 Then MsgBox "Please select Days and/or Amount.": Exit Sub
lr = Cells(Rows.Count, 1).End(3).Row
For rw = 3 To lr
If Cells(rw, 1) = ListBoxCustlist.Value Then dys = dys + Cells(rw, 2): amt = amt + Cells(rw, 3)
Next
If CheckBox1 Then msg1 = dys & " Days."
If CheckBox2 Then msg2 = Format(amt, "£#,###.00")
MsgBox msg1 & vbCr & msg2
End Sub