Problem with random code generation

Hi!

I have two files. One file contains products (Product.xls) and another file (Requests.xls) contains information about customers which select product with randomly generated code. When I input data in new row all OK. But if I add new column excel automatically recalculate randomly generated codes.

Please help me to correct this issue.
All files attached.

I use function Static_RAND :

Function Static_RAND(Lower As Double, Upper As Double) As Double
'Returns a random number which doesn't change when recalculated
'Between Lower and Upper (INCLUSIVE)
Static_RAND = Int((Upper - Lower + 1) * Rnd + Lower)
End Function