The route for Turkmenistan
First the rule, then the packing. Turkmenistan has not joined the Hague Apostille Convention, so document legalisation for Turkmenistan runs through the consular chain and not the short one. An apostille for Turkmenistan alone leaves the paper unfinished at the far end. The chain has two staffed stages, and that is exactly why sending your papers as one considered group beats sending them as they happen to come to hand.
What can share a journey
Think of the envelope as a queue ticket rather than a container. Everything inside it waits once and moves once, so the natural group is whatever you already hold in final form on the day you post. Civil records ordered together, a set of qualifications, papers about the same person for the same purpose: these belong in one envelope. Each sheet is still read and charged on its own, but they share the calendar rather than fighting over it.
The papers that should not join the group are the ones that are still becoming papers. A record you have applied for, a document with a spelling to correct, a sheet somebody else is signing this month. Holding an envelope open for a straggler is how an apostille from United Kingdom to Turkmenistan turns into two separate runs anyway, with the finished sheets sitting on a table gaining nothing while they wait.
How a batch moves
The stages are the same as for a single sheet, just wider. Sunderland Registration Service issues certified copies of births, marriages and deaths, and several can be ordered in one visit so your own certificates never leave home. The FCDO Legalisation Office then takes the envelope, checks each signature against the specimens it keeps, and posts the whole set back. It works by post only, so there is no counter where a batch could be handed in.
Keeping the set together at the far end
The consular desk usually wants the papers presented as a file rather than a trickle. A request built around one person and one purpose is easy to read; the same papers arriving on three different days are not. So an apostille issued in Sunderland for use in Turkmenistan is best thought of as one item in a bundle you assemble on this side. Keep the set intact from the moment it comes back from Milton Keynes.
The arithmetic of sending a group
Fees follow the sheets and time follows the envelope, and that difference is the whole point. Each document carries its own £45 standard charge, or £40 under the next-day tier open to registered businesses, no matter how they are packed. The waiting, by contrast, is shared. Five sheets posted on one morning come back on one morning, while the same five posted across a month keep the matter open for that whole month.