shielag25 Anti-bill shock for GoSurf 999 and below is 1,500. In your case, your internet usage should be charged 999 and an additional 501 (deducted from available PV first, on top of your bill second) according to GoSURF | HELP AND SUPPORT.
A scenario that would result to P2,500 is a Plan 999 with a GoSurf 999 Booster [999 + 999 GoSurf Booster + 501 for Anti-Bill Shock = 2,499]. Here, Boosters are automatically charged on top of the bill.
You may want to call the hotline and speak to a different CSR for a second opinion.