fn pay(
env: soroban_sdk::Env,
invoice_key: soroban_sdk::BytesN<32>,
buyer: soroban_sdk::Address,
merchant: soroban_sdk::Address,
amount: i128,
expires_at: u64,
) -> Result
fn get_config(env: soroban_sdk::Env) -> Config
fn get_receipt(
env: soroban_sdk::Env,
invoice_key: soroban_sdk::BytesN<32>,
) -> Result
fn __constructor(
env: soroban_sdk::Env,
registrar: soroban_sdk::Address,
usdc_sac: soroban_sdk::Address,
)