Whilst scouring the voucher sites I came across this in the fine print:
Qualifying Criteria: .... singles officially registered with single status are allowed...
What is "officially registered...single status" in Thailand?
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I would assume that it means exactly what it says.
Many (most) countries require all residents to register themselves with the country's authorities. The amount of data collected for each resident of course differs from country to country, but often includes information about close relatives (e.g. children or parents) and civil status (e.g. single, married or widowed).
If you live in a country with no such resident register, you must probably ask the operator how to interpret the condition. If you have no means to register yourself due to lack of such a register, you are strictly speaking not fulfilling the requirement.
For arranging a wedding in Thailand, the following clause appears:
Both parties need to provide ‘single status statements’ issued by their local registry office in their home country, or statement of divorce if that is the case.
Since the vouchers you are talking about are the kind of thing where you sit through a high pressure sales presentation in order to get a discount, you might want to ask the vendors what exactly it means and what must be provided to meet the letter of the agreement.