The only reference I can see on the AP website is that "Some business credit account customers have different size and weight restrictions" - I only use standard AP services and don't know if larger parcel sizes are allowed for account holders or eParcel users (and it's not mentioned on the site if you look for info). Best bet would be to ask the sellers how they are posting their items.

Registered is no longer available for standard parcel, and has been replaced with signature on delivery for an additional $2.95. Most standard parcel services come with basic tracking, and services like Click & Send and eParcel provide documentation the qualifies as proof of shipping for PayPal.