Zen Cart 1.5.7b
OPC 2.3.7-beta2 (2021-01-17)
PHP Version: 7.1.33
Shipping modules used: Storepickup, Advanced Shipper

Any suggestions as to why this message is frequently displayed when going through checkout?

"It's taking a little longer than normal to update your order's shipping cost. Please close this message and try again. If you continue to receive this message, please contact us."

Sometimes it has to be closed 2 or 3 times before you can actually complete checkout.