Functions and Triggers in PostgreSql

As I am new to DBs, I am learning PostgreSql and for a sample, I am trying to small scenario on Mobile Recharge Database System. The below is the query I have. I want to know what is the problem with the function I have written which should only return balance amount of an account number which is nothing but the customer id.

And, I also want to add the value in the wallet table when we add some value (that's topup kind of).

Please help me in this. Thanks.

The below is the complete query:

CREATE DATABASE "RECHARGESYS"
  WITH OWNER = postgres
       ENCODING = 'UTF8'
       TABLESPACE = pg_default
       LC_COLLATE = 'English_United States.1252'
       LC_CTYPE = 'English_United States.1252'
   CONNECTION LIMIT = -1;

--SERVICEPROVIDER TABLE:
DROP TABLE SERVICE_PROVIDERS;

CREATE TABLE SERVICE_PROVIDERS
(
SPID VARCHAR(5) PRIMARY KEY CHECK(SPID LIKE 'S%'),
SPNAME VARCHAR(50)
);

--CUSTOMER TABLE:
DROP TABLE CUSTOMER;

CREATE TABLE CUSTOMER
(
CID INT PRIMARY KEY,
CNAME VARCHAR(50)
);

--RECHARGE TABLE:
DROP TABLE RECHARGE;

CREATE TABLE RECHARGE
(
RID INT PRIMARY KEY,
CID INT REFERENCES CUSTOMER(CID),
SPID VARCHAR(5) REFERENCES SERVICE_PROVIDERS(SPID) CHECK(SPID LIKE 'S%'),
RENUMBER BIGINT,
AMOUNT INT
);

--TRANSACTION TABLE:
DROP TABLE TRANSACTION;

CREATE TABLE TRANSACTION
(
TID INT PRIMARY KEY,
SPID VARCHAR(5) REFERENCES SERVICE_PROVIDERS(SPID) CHECK(SPID LIKE('S%')),
RID INT REFERENCES RECHARGE(RID)
);

--WALLET TABLE:
DROP TABLE WALLET;

CREATE TABLE WALLET
(
WID INT PRIMARY KEY,
CID INT REFERENCES CUSTOMER(CID),
WAMOUNT INT
);

INSERT INTO SERVICE_PROVIDERS VALUES ('S1001', 'AIRTEL');
INSERT INTO SERVICE_PROVIDERS VALUES ('S1002', 'AIRCEL');
INSERT INTO SERVICE_PROVIDERS VALUES ('S1003', 'TATA DOCOMO');
INSERT INTO SERVICE_PROVIDERS VALUES ('S1004', 'IDEA');
INSERT INTO SERVICE_PROVIDERS VALUES ('S1005', 'VODAFONE');

SELECT * FROM SERVICE_PROVIDERS;

INSERT INTO CUSTOMER VALUES('20001','AHMED');
INSERT INTO CUSTOMER VALUES('20002','ASIF');
INSERT INTO CUSTOMER VALUES('20003','AHSRAF');
INSERT INTO CUSTOMER VALUES('20004','MAHESH');
INSERT INTO CUSTOMER VALUES('20005','ARUN');

SELECT * FROM CUSTOMER;

INSERT INTO WALLET VALUES('30001','20001','1000');
INSERT INTO WALLET VALUES('30002','20002','1000');
INSERT INTO WALLET VALUES('30003','20003','1000');
INSERT INTO WALLET VALUES('30004','20004','1000');
INSERT INTO WALLET VALUES('30005','20005','1000');


SELECT * FROM WALLET;

--IN THIS FUNCTION I WANT TO CHECK THE BALANCE ONCE I GIVE THE ACCOUNT NUMBER / CUSTOMER ID (ID):

CREATE OR REPLACE FUNCTION BALANCE(ACCNO INT)
RETURNS INT AS $BAL$
BEGIN
SELECT WAMOUNT INTO BAL FROM WALLET WHERE CID=ACCNO;
RETURN(BAL);
END;
$BAL$ LANGUAGE plpgsql;

SELECT BALANCE('20001') FROM WALLET;

--ALSO I WANT TRIGGER THAT CAN ADD AMOUNT TO THE WALLET.