Suitelet payment page integration.
You will need to learn SuiteScript. You can find documentation in the Help under SuiteCloud Platform > SuiteScript > SuiteScript Developer Guide. NetSuite also offers training programs under its website.
Once you have mastered basic SuiteScript you can turn your attention to Suitelets, paying particular attention top concurrency governance, here: SuiteCloud Platform > SuiteScript > SuiteScript 2.0 > SuiteScript 2.0 Script Types > SuiteScript 2.0 Suitelet Script Type