MySQL Multi-User / Table Locking

[TURKINATOR]

Hi guys, Im using MySQL 5.0.

Here is my two tables.

CREATE TABLE IF NOT EXISTS Invoice
(
InvoiceID	             MEDIUMINT	NOT NULL	AUTO_INCREMENT,
InvoiceDate	DATE		NOT NULL,
ClientID	             VARCHAR(7)	NOT NULL,
Total		INTEGER(10,2)	NOT NULL,
AmountPaid	INTEGER(10,2)	NOT NULL,
PRIMARY KEY	(InvoiceID),
FOREIGN KEY	(ClientID)	             REFERENCES	Clients(ClientID)
);

CREATE TABLE IF NOT EXISTS Transaction
(
InvoiceID	             MEDIUMINT	NOT NULL,
ProductID	VARCHAR(7)	NOT NULL,
QTY		INTEGER(2)	NOT NULL,
FOREIGN KEY	(InvoiceID) 	REFERENCES	Invoice(InvoiceID),
FOREIGN KEY	(ProductID)	REFERENCES	Products(ProductID)
);

The purpose of this is to record 1 InvoiceID to refer to multipul transaction items. So heres the problem. What if 2 transactions occured, then the below code will not work.

INSERT INTO Invoice (InvoiceDate, ClientID, Total, AmountPaid)
VALUES (NOW(), 'OZT0001', 130.00, 50.00);

INSERT INTO Transaction (InvoiceID, ClientID, ProductID, QTY, Total)
VALUES (LAST_INSERT_ID(), 'RAM0001', 2);

Is there a better way to construct this or any advise on how to go about this?