Ok you've asked Val
Depending on the level of expenditure involved. Not too expensive. Always well received. Those portraits of the King and Queen you see on the wall, almost obligatory in shops.If you think they may already have that, the same but featuring the late King Rama 5.
Alternatively a basket of flowers in an arrangement, these can be either real or artificial on which you attache a greeting card with your good wishes.This is the traditional gift, you will see many on display.The size of which depends on your budget.
As you mention they are Chinese merchants, but these would not be applicable to Thai.A matching pair of ceramic elephants to grace each side of the entrance, the alternative to these is the large flower urns, ranging from about 12" to 36" high.
You might also consider a basket of food goodies, prominent amongst which is the obligatory bottle of Black label. These are however more common at time of the new year, but are given on many occassions.The whiskey being the only farang item given unless the gift is from a farang personally then it would be okay.
Presumably they will be having the opening blessed by monks, if so and you will be present at the ceremony then an offering would be in order to the monks as well, normally food.
Have a good day and remember to take your shoes off before entering, yes even a shop on this occassion and when sitting be careful not to point your feet at the monks or the buddha image and if possible not at other people although most Thais are understanding of our difficulties in sitting and will not take offense.
The important thing to remember is, unlike the West, the gift doesn't have to be functional but has to bring good luck to the business. On one occassion I was given a frog with a gold coin in its mouth, this was thought to ensure I would always be receiving money.Think you get the idea?
