I need someone to write php scripts for the Infusionsoft API

General information for the business: Infusionsoft Script Development
Description of requirements/functionality: Here is what I need:

I have 3 Opportunity Custom Fields:

1 - Invoice Number
2 - Invoice Amount
3 - Invoice Due Date

What I would like to do, is trigger an email, say, 7 days before the Invoice Due Date, 1 Day Before the Invoice Due Date and one on the Due Date.

Very much like the field timer in the Campaign Builder, but referencing an Opportunity Custom Field (Which is not available in the CB)

I would also need this to be flexible, so I can change the timing, for instance:

If I can have a NumDays field in the HTTP Post, so I can set that to -7 to send 7 days before, or -3, to send 3 days before.

Happy to use either a Cronjob or HTTP Post.
Specific technologies required: php, Infusionsoft
Extra notes: