I just had someone in the community test this on GrapheneOS, and it worked fine for them.
They used a $10 USD card and confirmed that the balance was added. They stressed that they didn't use a VPN, but that's about it aside from that. I'm not entirely sure what, but it looks like the issue is not with Sandboxed Google Play, but something in your setup triggering Google's scam detection.
The tester's setup: p6 021000 | play store 34.5.10-21 [PR] 508814926 | play services 23.06.15 (190400-509321727) | gmscompatconfig 34