Attribute setup - can this be possible?

Hi, I am selling a door lock and matching key service.

The matching key service I offer to customer need to purchase 2 or more lock set.
BUT, customer just need to pay for one mtaching key servie out of 2 lock set purchase. If customer purchase 3 locks set, then pay 2 key matching service, buy 4 lockset, pay 3 key matching service, etc..

so, here is the situation I have.

lock price: 100
option: Key matching service
No: no charge
Yes: Add $30

so when customer add 2 lock set: 100x2=$200, the customer just need to pay "one key matching service which is $30"
but, current system add $60 when 2 item is add to cart and the key matching service option is selected.

How to achieve this without adding many key matching option in the product attrabute, like adding
1 keymatching service: add $30
2 keymatching service: add $60
1 keymatching service: add $90
1 keymatching service: add $120
etc, etc.

I have seen price factor and offset stuff in attrbute controller, but seems it's not the way to solve my problem.

Thanks in advance.

It's not clear from your post whether a customer can buy a single lock set and not need the key matching service, or whether they can buy two or more lock sets and choosing not to have matching keys. It's probably necessary to know if these are possible before a recommendation about approach can be made.

thanks Kuroi
yes, customer can buy only a single lockset, can buy 2 or more lockset without adding key matching service. Key matching service is optional depends on customers need. Hope I explain clearly.
Please share if you know how to solve my problem, thanks in advance.

kuroi:

It's not clear from your post whether a customer can buy a single lock set and not need the key matching service, or whether they can buy two or more lock sets and choosing not to have matching keys. It's probably necessary to know if these are possible before a recommendation about approach can be made.

I want to explain more here to make clear

thanks Kuroi
yes, customer can buy only a single lockset, can buy 2 or more lockset without adding key matching service. Key matching service is optional depends on customers need.

I set up my product as follow
Product
Price: $100
Attribute
Keymatching Service:
No
Yes add $30
When customer purchase 2 locksets and want to have the keymatching service.
2 key matching service which is $60 in this case is added to cart along with the product price.
100x2=200 + 2x30 = $260
But only one key matching service need to be added when 2 lockset is purchased,
so, it should be
100x2=200 + 1x30 = $230
If buy 3 locks and customer require key matching service
it should be
100x3=300 + 2x30 = $360

Hope I explain clearly.
Please share if you know how to solve my problem, thanks in advance.

keenskin:

thanks Kuroi
yes, customer can buy only a single lockset, can buy 2 or more lockset without adding key matching service. Key matching service is optional depends on customers need. Hope I explain clearly.
Please share if you know how to solve my problem, thanks in advance.