Update on this Issue
This issue appears to be related to using the taxable total of the cart in the shipping calculation like so:
//eCart Rule
function WAEC_eCart1_OverseasShipping() {
$totalShipping = 0;
if (true && ((sizeof($this->Items) > 0) && (isset($_SESSION['eCartCheckoutForm_shipping_country'])) && ((isset($_SESSION['eCartCheckoutForm_shipping_country'])?$_SESSION['eCartCheckoutForm_shipping_country']:"") != "GB") && ($this->GetTaxableTotal() <= 74.99))) {
$totalShipping += 7.50;//Result
}
return WA_eCart_FormatNumber($totalShipping, $this->ForceDecimalsC, $this->DecimalPlacesC);
}
//eCart Rule
whereas using the total of a column of the cart like so:
//eCart Rule
function WAEC_eCart1_BaseUKShipping() {
$totalShipping = 0;
if (true && ((sizeof($this->Items) > 0) && (isset($_SESSION['eCartCheckoutForm_shipping_country'])) && ((isset($_SESSION['eCartCheckoutForm_shipping_country'])?$_SESSION['eCartCheckoutForm_shipping_country']:"") == "GB") && ($this->TotalColumn("TotalPrice") <= 49.99))) {
$totalShipping += 2.50;//Result
}
return WA_eCart_FormatNumber($totalShipping, $this->ForceDecimalsC, $this->DecimalPlacesC);
}
//eCart Rule
works without an issue.