PayInvoice.cs

public static class PayInvoice
{
    public sealed record Command(Guid InvoiceId, string PaymentMethodToken);

    public sealed class Handler
    {
        private readonly AppDbContext _db;
        private readonly StripeClient _stripe;

        public Handler(AppDbContext db, StripeClient stripe)
        {
            _db = db;
            _stripe = stripe;
        }

        public async Task Handle(Command cmd)
        {
            var invoice = await _db.Invoices.FindAsync(cmd.InvoiceId)
                          ?? throw new InvalidOperationException("Invoice not found");

            var payment = new Payment(Guid.NewGuid(), invoice.Id, invoice.Total);
            _db.Payments.Add(payment);

            // Provider detail stays here, in the slice.
            var chargeResult = await _stripe.ChargeAsync(
                amount: invoice.Total.Amount,
                currency: invoice.Total.Currency,
                token: cmd.PaymentMethodToken);

            if (chargeResult.Succeeded)
            {
                payment.MarkSucceeded();
                invoice.ApplyPayment(invoice.Total);
            }
            else
            {
                payment.MarkFailed();
            }

            await _db.SaveChangesAsync();
        }
    }
}