Lovable edge function says 'Edge Function returned a non-2xx status code' and nothing else

The client error is deliberately vague; the real reason is in the function logs (dashboard → Edge Functions → your function → Logs / Invocations). Common ones with Lovable-generated functions:

• RESEND_API_KEY is undefined: the secret has a different name than Deno.env.get(...) reads.
• CORS: the browser sends an OPTIONS preflight first. The function must answer it:

if (req.method === "OPTIONS") {
  return new Response("ok", { headers: corsHeaders });
}

• An exception because the request body is missing a field.

To see the error message in your app, read it from the response:

const { data, error } = await supabase.functions.invoke("send-invoice", { body });
if (error) console.error(await error.context.json());

And have the function return { error: e.message } with status 400/500 in its catch block, so there's something to read.