Help required to Purchase Order Approval

Hi,

The requirement is to approve/release Purchase Order at header level based on total order value.

If total order value is less than 25,000 level 1(Release Code 10), then level 2(Release Code 20) is required .

If total order value is less than 1,00,000 level 3(Release Code 30), then level 4(Release Code 40) approval is required.

If total order value is greater than 1,00,000 level 1(Release Code 50), then level 2(Release Code 60) approval is required.

After creating Purchase order, mail should triggered to First Release Code. After approval of first release code, then mail should triggerred to Second Release Code.

But while i am saving purchase order, mail is triggering to both of release code. It should not happen.

Kindly suggest.