Scammed

Yea, the title says it all about today's shopping.

Felt so guilty lying to my family members.

How?
Zhiyi, Kuei Hsien and I went to this shop in Far East.
We tried on some clothes and pants.
The person mumbled about alterations to the pants so Zhiyi and I just mumbled 'ya'.
In lightning speed, the person send the two pants for alteration although we didn't even pay for them.
In the end, since everything there, yes, EVERYTHING there were expensive, we said we don't want to buy.
BUT, the person said the pants already sent for alteration, cannot reject le, must pay FULL amount, $70!!!
Of course we were not happy.
But end up like no choice so we bought those damn expensive and normal looking pants.

Damn sianz, and my mum was of course angry, so I calmed her by lying that the pants discount until $40.
Then, when I reached home I asked her how she felt.
She said since it's $40, it's still considered ok.
She thus limited my budget of the clothes to $40, and shoes to $40.
Then she continued by saying that if the pants cost the original price of $70, she will flare up and tear away the pants immediately.
So now I am feeling very guilty lying to her.
But how?
I know that one lie would lead to more lies, but if I told her the truth, God knows what will happen.

I am in a dilemma.
What must I do?
I am an unfilial and bad person to lie to my mum.