Sadly the checkout dialog has CORS violations that they probably don't care to fix.
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at ‘https://payment-website-pci.ol.epicgames.com/purchase/xsrf?purchaseToken=XXX&flow=PURCHASE’. (Reason: Credential is not supported if the CORS header ‘Access-Control-Allow-Origin’ is ‘*’).
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at ‘https://talon-service-prod.ecosec.on.epicgames.com/v1/init’. (Reason: Credential is not supported if the CORS header ‘Access-Control-Allow-Origin’ is ‘*’).
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at ‘https://bam.nr-data.net/1/93a8bd5691?a=27815142&v=1.249.0&to=MlxXbUBZWkJUAkVQCgsWcktTUVhCdg5asdfKVVlcQBdEUEwMVFcRSklUQF9dWkU%3D&rst=1610&ck=0&s=67133c36f4b2d060&ref=https://store.epicgames.com/purchase&ap=271&be=712&fe=717&dc=347&fsh=0&perf=%7B%22timing%22:%7B%22of%22:1703387184928,%22n%22:0,%22f%22:712,%22dn%22:712,%22dne%22:712,%22c%22:712,%22s%22:712,%22ce%22:712,%22rq%22:712,%22rp%22:712,%22rpe%22:712,%22xx%22:1056,%22ds%22:1057,%22de%22:1059,%22dc%22:1428,%22l%22:1428,%22le%22:1429%7D,%22navigation%22:%7B%7D%7D&fcp=963’. (Reason: Credential is not supported if the CORS header ‘Access-Control-Allow-Origin’ is ‘*’).
Probably can use it to feed the ttw mod for new Vegas that combines the two into a single game. I used gog versions to install the mod on steam deck and uninstalled fo3 after.
I recommend a significantly stronger CPU computer for the TTW installer though as it converts the audio to be the same format as nv and it took like 5 days on the deck.