satellite/payments: conditionally add saved trees info to stripe invo…

…ices footer

Added logic to calculate saved trees count during invoice generation.
Added saved trees info to invoices footer if number of saved trees is more than 0.

Issue:
#6694

Change-Id: Ifca1cc9ec8bc15b0a589040c951e1ff273aaf62b

satellite/payments/stripe/service.go

@@ -1145,12 +1145,25 @@ func (service *Service) createInvoice(ctx context.Context, cusID string, period
 		}
 
 		whitePaperLink := "https://www.storj.io/documents/storj-sustainability-whitepaper.pdf"
-		footer = stripe.String(fmt.Sprintf(
+		footerMsg := fmt.Sprintf(
 			"Estimated Storj Emissions: %.3f kgCO2e\nEstimated Hyperscaler Emissions: %.3f kgCO2e\nMore information on estimates: %s",
 			impact.EstimatedKgCO2eStorj,
 			impact.EstimatedKgCO2eHyperscaler,
 			whitePaperLink,
-		))
+		)
+
+		savedValue := impact.EstimatedKgCO2eHyperscaler - impact.EstimatedKgCO2eStorj
+		if savedValue < 0 {
+			savedValue = 0
+		}
+
+		savedTrees := service.emission.CalculateSavedTrees(savedValue)
+		if savedTrees > 0 {
+			treesCalcLink := "https://www.epa.gov/energy/greenhouse-gases-equivalencies-calculator-calculations-and-references#seedlings"
+			footerMsg += fmt.Sprintf("\nEstimated Trees Saved: %d\nMore information on trees saved: %s", savedTrees, treesCalcLink)
+		}
+
+		footer = stripe.String(footerMsg)
 	} else {
 		itemsIter := service.stripeClient.InvoiceItems().List(&stripe.InvoiceItemListParams{
 			Customer: &cusID,