order and order line item

Suppose we have the usual order & order line item entities:

CREATE TABLE "order" (
id INT PRIMARY KEY,
date DATE NOT NULL
);

CREATE TABLE orderlineitem (
id INT PRIMARY KEY,
orderid INT REFERENCES "order"(id),
seq INT NOT NULL,
CONSTRAINT con1 UNIQUE (orderid, seq),
productid INT NOT NULL,
quantity INT NOT NULL,
price NUMERIC(18,4) NOT NULL
);

how do we enforce things like, for example: a) each order must contain
at least 1 line item; or b) each order must have a non-zero total value;
without making INSERT painful? I'm new to triggers, so I'm thinking of
something like a "before COMMIT" trigger but there's no such thing
apparently.

Regards,
dave