Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE stores ( store_id text, address_id number, marketing_region_code text, store_name text, store_phone text, store_email_address text, other_details text )
TABLE: CREATE TABLE performers ( performer_id number, address_id number, customer_name text, customer_phone text, customer_email_address text, other_details text )
TABLE: CREATE TABLE performers_in_bookings ( order_id number, performer_id number )
TABLE: CREATE TABLE ref_payment_methods ( payment_method_code text, payment_method_description text )
TABLE: CREATE TABLE bookings ( booking_id number, customer_id number, workshop_group_id text, status_code text, store_id number, order_date time, planned_delivery_date time, actual_delivery_date time, other_order_details text )
TABLE: CREATE TABLE products ( product_id text, product_name text, product_price number, product_description text, other_product_service_details text )
TABLE: CREATE TABLE clients ( client_id number, address_id number, customer_email_address text, customer_name text, customer_phone text, other_details text )
TABLE: CREATE TABLE invoices ( invoice_id number, order_id number, payment_method_code text, product_id number, order_quantity text, other_item_details text, order_item_id number )
TABLE: CREATE TABLE addresses ( address_id text, line_1 text, line_2 text, city_town text, state_county text, other_details text )
TABLE: CREATE TABLE bookings_services ( order_id number, product_id number )
TABLE: CREATE TABLE customers ( customer_id text, address_id number, customer_name text, customer_phone text, customer_email_address text, other_details text )
TABLE: CREATE TABLE marketing_regions ( marketing_region_code text, marketing_region_name text, marketing_region_descriptrion text, other_details text )
TABLE: CREATE TABLE invoice_items ( invoice_item_id number, invoice_id number, order_id number, order_item_id number, product_id number, order_quantity number, other_item_details text )
TABLE: CREATE TABLE order_items ( order_item_id number, order_id number, product_id number, order_quantity text, other_item_details text )
TABLE: CREATE TABLE services ( service_id number, service_type_code text, workshop_group_id number, product_description text, product_name text, product_price number, other_product_service_details text )
TABLE: CREATE TABLE ref_service_types ( service_type_code text, parent_service_type_code text, service_type_description text )
TABLE: CREATE TABLE drama_workshop_groups ( workshop_group_id number, address_id number, currency_code text, marketing_region_code text, store_name text, store_phone text, store_email_address text, other_details text )
TABLE: CREATE TABLE customer_orders ( order_id number, customer_id number, store_id number, order_date time, planned_delivery_date time, actual_delivery_date time, other_order_details text )

### Question ###
Find the payment method that is used the most often in all the invoices. Give me its code.

### Accurate SQL ###

SELECT payment_method_code FROM invoices GROUP BY payment_method_code ORDER BY COUNT(*) DESC LIMIT 1