Recently PayPal has updated its services to require TLS 1.2 for all HTTPS connections, this has caused any Paypal Express payment request that is sent from a non TLS 1.2 protocol will return error msg below :
This transaction is invalid. Please return to the recipient's website to complete your transaction using their regular checkout flow.
If you have such issue, then please follow the solution below.
For v8 site :
1. Apply the TLS 1.2 patch first. (If you have already installed the TLS 1.2 patch before, then please skip to next step).
You can download at : http://www.vpasp.com/sales/addons800.asp and select "TLS 1.2 8.0" to download.
After downloaded, please install it to your v8 site.
2. Open your file paypalproexpress.asp and locate this code (around line 479) :
Shopxmlhttp checkxml_paypalec, x_GatewayLocation, anrequest, resultXML, "POST", xmlerror
dim rheadercnt, rheadertypeary(10), rheadervalueary(10)
rheadercnt = 0
Shopxmlhttp_v8 "auto", x_GatewayLocation, anrequest, resultXML, "POST", xmlerror, rheadercnt, rheadertypeary, rheadervalueary, "",""
Shopxmlhttp checkxml_paypalec, x_GatewayLocation, anrequest, resultXML, "POST", xmlerror
3. Please replace to :
dim rheadercnt, rheadertypeary(10), rheadervalueary(10)
rheadercnt = 0
Shopxmlhttp_v8 "auto", x_GatewayLocation, anrequest, resultXML, "POST", xmlerror, rheadercnt, rheadertypeary, rheadervalueary, "",""