To some degree what you can and can't do will depend on which section you're in!
Section A/B provides a lump sum as standard, while section C doesn't but gives you the option of giving up some pension to get one.
It's unclear from your post which one applies to you.
But the whole point of paying AVC's(Bonusplan & Flexiplan) is to fund the tax free lump sum(also known as the Pension Commencement Lump Sum), so you don't have to give up any or as much pension to get that lump sum.
So as a general rule always use your AVC's to fund that first and then if you want a bigger lump sum(up to 25% of your 'total pot value'), take it from the pension. But that will mean the pension will be less.
See page 12 of your plan guide for an explanation of the 'total pot value': https://www.royalmailpensionplan.co.uk/ ... ts/library
I hope that helps in some way.